Add announcement role (announcement@v1) skeleton

[marcelveldt]

Draft client-side reference implementation for the announcement@v1 role proposed in Sendspin/spec#126: short per-client audio clips (TTS, chimes, alerts) decoded on a second pipeline next to the media stream, with the ducking policy handed to the embedder. Opened alongside the spec PR so the discussion has a concrete client shape to poke at; it tracks the spec.

What's here

• Wire surface: announcement@v1 role id, AnnouncementSupportObject (formats + own buffer_capacity) in client/hello, stream/start.announcement parsing (codec params, a single required start_timestamp, media_duck_db 0-50 / duck_ramp_ms 0-2000 / optional volume / optional override_mute, out-of-range values dropped to defaults), client/state.announcement, and binary type 24 (SENDSPIN_ROLE_ANNOUNCEMENT = 6, routed through get_binary_role() unchanged). Chunks are untimed: the stream's single start time rides the codec header.
• AnnouncementRole following the add-a-role pattern, with an AnnouncementRoleListener the embedder implements: on_announcement_write() (PCM, task thread) plus on_announcement_start/end() (main loop). on_announcement_start() carries the duck params, start_timestamp and override_mute (that's where e.g. ESPHome calls apply_ducking()); it also re-fires when the server re-sends stream/start to update config in place. on_announcement_end() fires on server end, local completion, abort, and transport loss alike, so media can never stay stuck ducked.
• AnnouncementTask: a dedicated lightweight decode task (own ring buffer + decoder, one extra thread). No sync machinery: output begins at the single start_timestamp via the time filter (or as soon as possible), which is also what aligns a coordinated multi-speaker start; chunks are never dropped for lateness and a stalled stream ends locally after 5 s.
• Lifecycle: stream/end with omitted roles ends announcements too; stream/clear is media-scoped and never touches the announcement stream (replacing a clip is a stream/end then a new stream/start).
• Build gate: SENDSPIN_ENABLE_ANNOUNCEMENT (Kconfig default n while the spec settles; depends on SENDSPIN_ENABLE_PLAYER, which supplies the ring buffer and decoder).
• Tests: protocol round-trip coverage (hello support, stream/start defaults/validation including the required start_timestamp and override_mute, client/state, binary allocation) plus a config-update-in-place test. Host build is clean under -Wall -Wextra -Wpedantic; the full suite passes.

Not here yet

• The ESPHome layer (a SendspinAnnouncementSource feeding the speaker_source pipeline plus duck-parameter plumbing) lives in the esphome repo and needs a released sendspin-cpp first.
• Gain handling: duck, announcement volume, and override_mute are applied by the embedder at the sink/mixer; the library only transports the parameters.

Marked draft until the spec PR settles.