Port upstream 0.66.0: shared Chromium LevelDB + Snappy reader

[Finesssee]

Summary

Adds a shared, read-only LevelDB reader for Chromium browser storage at rust/src/browser/leveldb/, with no new crate (user-approved: hand-written LevelDB log/table parser plus a Snappy block decoder). It is infrastructure only: nothing calls it yet, so there is no behavior change. It unblocks the two upstream 0.66.0 items that were deferred for lack of a LevelDB reader: Kimi Chromium local-storage access_token import (steipete#3923) and MiniMax browser-storage discovery (steipete#3883).

• read_entries(dir): scans *.log (write-ahead log) and *.ldb/*.sst (sorted tables, prefix-compressed blocks, none/Snappy compression), resolves each user key to its newest sequence, and drops deleted keys.
• local_storage::read_local_storage_entries(dir, origin): decodes Chromium Local Storage items (_<origin>\0<format byte><key>, Latin-1 or UTF-16LE text) for one exact origin. local_storage_dir(profile_dir) gives <profile>/Local Storage/leveldb.
• snappy::decompress(input, max_len): raw Snappy blocks with a caller-set output cap.
• Bounded and tolerant: files over 64 MiB and blocks inflating past 16 MiB are skipped; a truncated live-log tail keeps earlier records; a corrupt file or block is skipped with a tracing::debug line (no key or value content is logged).

Known limits (documented in the module docs): the MANIFEST is not consulted (a compacted-away table not yet deleted can still be read; newer sequences win), checksums are not verified, blocks with compression types other than none/Snappy are skipped, IndexedDB value decoding is out of scope.

Upstream reference

• Release bullet: Kimi "import web access tokens from Chromium local storage" (fix(kimi): import kimi.ai session from browser local storage steipete/CodexBar#3923, 49e7ff3e) and MiniMax "discover browser session storage" (fix(devin): share Chromium browser session discovery steipete/CodexBar#3883, 62cdd065), v0.66.0.
• Upstream reads LevelDB through SweetCookieKit's ChromiumLocalStorageReader.readEntries(for: origin, in: levelDBDirectory), an external Swift package, so there is no upstream reader source at the tag to copy. Consumer call sites at v0.66.0: Sources/CodexBarCore/BrowserLocalStorageAPI.swift, Sources/CodexBarCore/Providers/Kimi/KimiCookieImporter.swift (localStorageTokens), Sources/CodexBarCore/ChromiumLocalStorageDiscovery.swift.
• The on-disk formats implemented are the public LevelDB log/table formats and the Snappy format description; test fixtures are built from those layouts.

Ported / Deferred

Ported: the reader, Chromium Local Storage decoding, Snappy decoder.

Deferred to follow-up micro PRs (queue items 20 and 21):

• Kimi localStorageTokens (JWT filter, exp check, ordering after cookie sources); needs the Kimi region work from Integrate reviewed provider, history, and tray ports #610.
• MiniMax storage discovery across the browser catalog (Comet/Yandex are not in BrowserType) and IndexedDB origin-prefix scanning.
• Browser/profile enumeration helpers for consumers; detection::BrowserProfile.path plus local_storage_dir already covers Local Storage.
• Checking the reader against a live Chrome/Edge profile: not done in this PR (an automated read of the local browser profile was blocked by the sandbox policy). Fixtures are self-built from the format specs, so a real-profile smoke check is worth doing in the Kimi follow-up.

Validation

Toolchain cargo +1.98.0, slot-4 target dir, E-core wrappers.

• cargo +1.98.0 fmt --all: clean
• cargo +1.98.0 clippy --workspace --all-targets -- -D warnings: pass
• cargo +1.98.0 test -p codexbar leveldb -- --test-threads=4: 24 passed, 0 failed
• cargo +1.98.0 test -p codexbar browser:: -- --test-threads=4: 39 passed, 0 failed

Tests cover Snappy literal/copy-1/2/4 elements and malformed streams, log framing across block edges and padding, truncated tails, table prefix compression across blocks, Snappy-compressed blocks, bad footer/handle/compression, newest-sequence resolution across log and table, tombstones, and Local Storage Latin-1/UTF-16 decoding with exact-origin matching. I did not run the full cargo test -p codexbar (no shared code touched; only browser/mod.rs gained one pub mod line).

Affected areas

• Rust backend (rust/src/browser/)
• Tauri shell
• Frontend
• Settings / bridge types
• No new dependencies. No file over 1000 lines (largest: tests.rs, 625).

UI proof

Not applicable

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[Finesssee]

Thermo-nuclear review

Reviewed by Codex gpt-6-luna (xhigh); verified and validated by Claude

• Medium, rust/src/browser/leveldb/log.rs:82: a malformed write batch could emit earlier operations before a later one failed, and sequence numbers could wrap. Fixed: whole batch validated (including trailing bytes) and sequence range checked before emitting.
• Medium, rust/src/browser/leveldb/table.rs:94: malformed block entries were treated as normal end-of-block, letting partial records through. Fixed: entry errors propagate, restart array validated, records emitted only after the whole block decodes.
• Medium, rust/src/browser/leveldb/table.rs:126: corrupt block handles could read across table section boundaries, and uncompressed blocks bypassed the block size limit. Fixed: reads bounded by index/footer offsets, raw blocks capped at MAX_BLOCK_BYTES.
• Medium, rust/src/browser/leveldb/mod.rs:128: a file could grow after the size check and exceed the read limit. Fixed: read capped at limit + 1 byte and rejected if exceeded.
• Low, rust/src/browser/leveldb/mod.rs:16: docs implied the per-file limits bound total memory. Fixed: docs now state the snapshot grows with live data.

[Finesssee]

Follow-up: all 5 thermo review findings above are fixed in the "Address thermo review" commit. Nothing left open.

Commands run (E-core wrappers, Rust 1.98.0): cargo fmt --all -- --check, cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings (clean), cargo test --manifest-path rust/Cargo.toml --lib browser (47 passed). The change is confined to rust/src/browser/leveldb/, so the Tauri crate clippy and frontend tests were not re-run; no UI change.