feat: add message/channel paginator stack with thread minimal-init and state stores

CLAUDE.md

@@ -52,7 +52,7 @@ This is a single-package SDK with **no monorepo**. The public surface is everyth
 ### Module map of `src/`
 
 - **`client.ts` — `StreamChat` facade.** ~5k-line class. Prefer `StreamChat.getInstance(key, secret?, options?)` — the constructor exists for advanced uses but `getInstance` is what `connectUser` warnings and most docs assume. Owns: the axios instance, WS connection lifecycle, `TokenManager`, and a registry of subsystem managers (`threads`, `polls`, `notifications`, `reminders`, `moderation`, `uploadManager`, `messageDeliveryReporter`, plus an optional `offlineDb` injected via `setOfflineDBApi`). New REST endpoints are added here as methods that call `axiosInstance` and return a type from `types.ts`.
-- **`channel.ts` (~2.5k lines) + `channel_state.ts` (~1.1k) + `channel_manager.ts` + `channel_batch_updater.ts`** — per-channel object, its in-memory state, and the manager that orchestrates collections of channels (query/sort/filter, pagination, archived/pinned handling).
+- **`channel.ts` (~2.5k lines) + `channel_state.ts` (~1.1k) + `channel_manager.ts` + `channel_batch_updater.ts`** — per-channel object, its in-memory state, and the manager that orchestrates collections of channels (query/sort/filter, pagination, archived/pinned handling). **Messages are NOT stored on `channel.state`.** The message list, thread replies, and pinned messages each live in a paginator — `channel.messagePaginator`, `thread.messagePaginator`, and `channel.pinnedMessagesPaginator` — which are the single source of truth (interval storage + a canonical `ItemIndex`). Read them via `channel.messagePaginator.state.items` / `.getItem(id)` / `.headmostItem` (newest loaded item), and mutate via the paginator (`ingestItem` / `removeItem`), never a legacy `channel.state.addMessageSorted()` / `state.messages` (removed). `channel.state.last_message_at` was **removed**; the channel's latest-message timestamp lives on `channel.messagePaginator.lastMessageAt` (its `aggregateState` store — seeded from `ChannelResponse.last_message_at`, then advanced monotonically as messages are ingested). See `docs/breaking-changes-v14-v15.md`.
 - **`connection.ts` (`StableWSConnection`) + `connection_fallback.ts` (`WSConnectionFallback`)** — realtime transport. Primary WS implementation does its own 25s ping / 35s health-check loop and reconnects on close/error/offline events; the fallback long-polls over HTTP. The client picks between them based on first-connect outcome; both emit `connection.changed` / `transport.changed` events into the client's local event bus.
 - **`store.ts` — `StateStore`.** Reactive primitive (see "State and subscription patterns" below).
 - **`signing.ts` — webhook + token helpers.** Server-side primitives `verifyAndParseWebhook`, `parseSqs`, `parseSns`, `verifySignature` (recent CHA-3071 added compressed-payload support). These are re-exported through `client.ts`. **The HMAC is always computed over the uncompressed JSON bytes** — gzip detection uses the `1f 8b` magic bytes, not headers, so the same handler works whether your platform middleware auto-decompressed or not. `CheckSignature` is deprecated in favor of `verifySignature` purely to fix parameter order; new code should use `verifySignature(body, signature, secret)`.

docs/breaking-changes-v14-v15.md

@@ -0,0 +1,164 @@
+# Breaking Changes: v14 → v15 (stream-chat JS)
+
+Consumer-facing **breaking changes** on the v15 line of the JS SDK — the `message-paginator`
+initiative (`feat/message-paginator-master-merge` and the follow-on
+`refactor/remove-legacy-channelstate-storage`), beyond the master ⇄ PR merge itself. This is the
+source for the v14 → v15 migration guide / release notes. Append newest at the top. Keep each entry
+self-contained: what changed, before → after, how to migrate, why.
+
+> Scope note: this file tracks **public API / observable behavior** changes. Internal refactors with
+> identical output belong in `decisions.md`, not here.
+
+---
+
+## `ChannelState` message / thread / pinned storage removed — paginators are the source of truth
+
+**Area:** `ChannelState`, `Channel`, `utils` · **Status:** implemented
+
+The channel's messages, thread replies, and pinned messages are no longer stored on `channel.state`.
+Each list now lives in a paginator that is the single source of truth (interval storage + a canonical
+`ItemIndex`):
+
+- **Main message list** → `channel.messagePaginator`
+- **Thread replies** → `thread.messagePaginator` (via `client.threads` / the `Thread` object)
+- **Pinned messages** → `channel.pinnedMessagesPaginator`
+
+### Removed from `ChannelState`
+
+Properties / getters: `messages`, `latestMessages`, `messageSets`, `messagePagination`, `threads`,
+`pinnedMessages`. (Also `isUpToDate` and the `last_message_at` setter — see the dedicated entries
+below.)
+
+Methods: `addMessageSorted`, `addMessagesSorted`, `removeMessage`, `findMessage`,
+`findMessageByTimestamp`, `filterErrorMessages`, `loadMessageIntoState`, `clearMessages`,
+`initMessages`, `pruneOldest`, `addReaction`, `removeReaction`, `updateUserMessages`,
+`deleteUserMessages`, `addPinnedMessages`, `addPinnedMessage`, `removePinnedMessage`,
+`removeQuotedMessageReferences` (plus the internal `_updateMessage` / `_updateQuotedMessageReferences`
+/ `_add*`/`_remove*` reaction helpers).
+
+### Removed from `utils` (re-exported through the package root)
+
+`addToMessageList`, `messageSetPagination`, `binarySearchByDateEqualOrNearestGreater`,
+`deleteUserMessages`, and the `MessageSet` / message-set pagination types.
+
+### Migrate
+
+| Before (v14)                                                  | After (v15)                                                                          |
+| ------------------------------------------------------------- | ------------------------------------------------------------------------------------ |
+| `channel.state.messages`                                      | `channel.messagePaginator.state.items` (reactive) / `channel.messagePaginator.items` |
+| `channel.state.latestMessages`                                | `channel.messagePaginator.headItems` / `.lastMessage`                                |
+| `channel.state.messagePagination`                             | `channel.messagePaginator.state` (`hasMoreHead` / `hasMoreTail` / `cursor`)          |
+| `channel.state.threads[parentId]`                             | `thread.messagePaginator.state.items` (resolve the `Thread` via `client.threads`)    |
+| `channel.state.pinnedMessages`                                | `channel.pinnedMessagesPaginator.state.items`                                        |
+| `channel.state.addMessageSorted(m)` / `addMessagesSorted(ms)` | `channel.messagePaginator.ingestItem(m)`                                             |
+| `channel.state.removeMessage({ id })`                         | `channel.messagePaginator.removeItem({ id })`                                        |
+| `channel.state.findMessage(id)`                               | `channel.messagePaginator.getItem(id)`                                               |
+
+Reactive reads use `useStateStore(channel.messagePaginator.state, …)` (or the paginator's `state`
+store directly). Pin/unpin, reactions, user updates, and deletions are applied to the paginators by
+the SDK's own event handlers — application code should not call the removed mutators.
+
+### Why
+
+Messages were previously stored twice — a flat `ChannelState` list plus message-set/pagination
+bookkeeping — which had to be kept in sync with the paginators and could drift. Consolidating on the
+paginators removes the dual-write, gives one API/behavior across the main list, threads, and pinned
+messages (dedup-by-id, interval merge, `getItem`/`removeItem`, head-window semantics), and lets
+`last_message_at` be derived rather than separately maintained.
+
+## `ChannelState.last_message_at` removed — use `channel.messagePaginator.lastMessageAt`
+
+**Area:** `ChannelState` · **Status:** implemented
+
+`channel.state.last_message_at` was **removed entirely** (it was briefly a read-only getter earlier
+in v15; that getter is gone too). The channel's latest-message timestamp is now owned by the message
+paginator as a whole-collection aggregate.
+
+- **Before:** `channel.state.last_message_at` (writable, then a derived getter). Internally
+  maintained by the now-removed `Channel._trackLatestMessage`.
+- **After:** `channel.messagePaginator.lastMessageAt` — a `Date | null` **derived** getter over the
+  paginator's `aggregateState` store (`MessagePaginatorAggregateState = { lastMessage,
+seededLastMessageAt }`). It returns `max(lastMessage?.created_at, seededLastMessageAt)`:
+  `lastMessage` is the newest loaded/received message (advanced on ingest), `seededLastMessageAt` is
+  the server floor **seeded from `ChannelResponse.last_message_at`** (for channels whose newest
+  message isn't loaded). Deriving the sort key from the two independent facts means it can never drift
+  from the display message. Subscribe to `channel.messagePaginator.aggregateState` for reactivity.
+- **Migrate:** replace `channel.state.last_message_at` reads with
+  `channel.messagePaginator.lastMessageAt`. It is not writable; the value is derived from ingested
+  messages and the server seed.
+- **Why:** the message paginator is the single source of truth for messages; `last_message_at` is an
+  aggregate over them (the dual of pagination). Deriving it through a `ChannelState` getter that
+  reached into the paginator's message index risked stale/mixed-basis sorting (a seeded-but-stale
+  paginator preferred over a fresher server value); a single seeded-then-advanced value on the
+  paginator removes that hazard.
+- **Tracking relocated:** `MessageIntervalPaginator`'s `state.latestMessageId` and the `latestMessage`
+  getter (id resolved from the pagination `state`) were replaced. The tracked latest now lives on
+  `MessagePaginator.aggregateState.lastMessage`, advanced on every ingest.
+  `MessagePaginator.lastMessage` remains as a convenience getter but now reads `aggregateState`.
+  This matters for reactivity: pagination `state` only emits when the **active** interval is impacted,
+  so a WS message landing in the (non-active) head interval would not notify a `state`-derived
+  latest; `aggregateState` is written directly on each advance and emits regardless — subscribe to it
+  (e.g. for a channel/thread list item's latest-message display). `aggregateState.lastMessage` is a
+  LIVE reference: refreshed in place on edit/soft-delete/reaction of the current latest and recomputed
+  on hard-remove, and it honors `skip_last_msg_update_for_system_msgs` (system messages neither
+  reorder a channel nor become its displayed latest). A consumer that previously showed the unfiltered
+  newest message (`headmostItem`) as the channel-list preview will now skip system messages under that
+  config — a deliberate behavior change so the preview and the channel's sort position agree.
+
+## Newest-loaded window is exposed as computed getters `headItems` / `headmostItem`
+
+**Area:** `BasePaginator` · **Status:** implemented
+
+The newest-loaded window is exposed as **computed getters** on the paginator — `paginator.headItems`
+(the window; `[]` before the first load) and `paginator.headmostItem` (its single newest item),
+derived from the intervals on read.
+
+- **Migrate:** the v14 `channel.state.latestMessages` reactive array becomes
+  `channel.messagePaginator.headItems`. Because these are getters, **not** `PaginatorState` fields,
+  they are not subscribable via `useStateStore(paginator.state, (s) => s.headItems)` — read them
+  directly, or subscribe to `channel.messagePaginator.aggregateState` for the reactive last-message
+  signal.
+- **Why:** the value is derivable from the intervals on demand, so it does not need to be materialized
+  (and re-emitted) into pagination state.
+
+## `Channel.lastMessage()` removed — use `channel.messagePaginator.headmostItem`
+
+**Area:** `Channel` · **Status:** implemented
+
+`channel.lastMessage()` was removed. It returned the newest loaded message (the head edge of the
+message paginator's latest window); read `channel.messagePaginator.headmostItem` directly instead.
+
+- **Migrate:** `channel.lastMessage()` → `channel.messagePaginator.headmostItem`.
+- **Note:** `headmostItem` is the newest _loaded_ message, **unfiltered** (includes system messages) —
+  distinct from `channel.messagePaginator.lastMessage`, the filtered chronological latest (honors
+  `skip_last_msg_update_for_system_msgs`) that backs `lastMessageAt`. Use `headmostItem` for "the newest
+  message on screen"; use `lastMessage` / `lastMessageAt` for channel-list ordering.
+- **Why:** it was a thin wrapper over `headmostItem`, and sharing the name `lastMessage` with the
+  differently-filtered paginator getter was misleading.
+
+## `ChannelState.isUpToDate` / `setIsUpToDate` removed
+
+**Area:** `ChannelState` · **Status:** implemented
+
+The `isUpToDate` flag and its `setIsUpToDate(boolean)` setter were removed. They gated whether an
+incoming `message.new` was appended to the visible message list when the user had scrolled to older
+history.
+
+- **Before:** UI SDKs set `channel.state.setIsUpToDate(false)` when jumping to an older window so live
+  messages were not forced onto the visible list, and read `channel.state.isUpToDate` to decide
+  whether to show a "jump to latest" affordance.
+- **After:** neither exists. Message routing is handled structurally by the message paginator: a live
+  message newer than the loaded head lands in the head (or logical-head) interval, which is not the
+  active window when the viewer has jumped away, so the visible window is preserved with no flag.
+- **Migrate:**
+  - "Am I viewing the newest window?" → `channel.messagePaginator.isActiveIntervalAtHead` (getter).
+  - "Are there newer messages not yet loaded?" → `channel.messagePaginator.hasMoreHead` (reactive
+    via `channel.messagePaginator.state`).
+  - "Jump to the latest" → `channel.messagePaginator.jumpToTheLatestMessage()`.
+- **Behavioral note:** `last_message_at` now advances on every incoming message regardless of the
+  viewer's scroll position (the old `isUpToDate` suppression is gone) — it is a channel-level fact,
+  independent of what the UI is currently viewing.
+- **Why:** the flag duplicated state the paginator already models, and was only ever set `false` on
+  disconnect on this branch — its message-list responsibility had already moved to the paginator.
+
+---