feat(huddle): huddle-transcript signed event kind + SDK builder (#2513)

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Summary

A huddle's spoken content is the one modality that never becomes part of the event log. Agents watching a channel can see that a huddle happened (lifecycle events), but never what was said. This PR adds the protocol foundation for huddle transcripts as first-class, searchable, signed channel events — the core of #2513.

This is scoped as the reviewable protocol slice: the event kind, a typed reader, an SDK builder, and relay ingest registration — all built and tested. The server-side recording + transcription pipeline is designed below and called out as the honest follow-up, so this PR is small and correct rather than large and unbuildable.

What's in this PR

• buzz-core/src/kind.rs — new KIND_HUDDLE_TRANSCRIPT = 48104, placed in the huddle lifecycle range (48100–48106) and registered in ALL_KINDS (so the duplicate-detection test covers it).
• buzz-core/src/huddle.rs — HuddleTranscriptSegment, the reader side agents use. A segment's content is plain transcript text (so it is FTS-indexed and readable exactly like a message); structure lives in tags:

  kind:    48104
  content: "the quick brown fox"        ← plain text, FTS-indexed
  tags:    ["h", <channel-uuid>]        ← channel scope (NIP-29 group)
           ["p", <speaker-pubkey>]      ← attribution
           ["e", <huddle-started-id>]   ← links segment → huddle session
           ["start", <ms>] ["end", <ms>]← offsets from huddle start
           ["lang", <bcp47>]            ← optional
  

  from_content_and_tags parses a segment back out (used by agents / any consumer).
• buzz-sdk/src/builders.rs — build_huddle_transcript(...), the signed-event builder, with validation (64-hex speaker + huddle id, non-empty text, 16 KB cap, end >= start).
• buzz-relay/src/handlers/ingest.rs — registers the kind's write scope (ChannelsWrite) and h-channel scoping, alongside the other huddle events.

Why this shape

• Attribution is free server-side. The audio room already keys every participant by (pubkey, peer_index) (crates/buzz-relay/src/audio/room.rs), so a segment maps to a speaker pubkey with no diarization.
• Searchable with no new indexer. Because content is plain text, transcripts flow through the existing Postgres FTS search_tsv generated column automatically.
• Readable by agents like any message — a channel-member agent gets huddle content as ordinary context, closing the "incident memory" gap in the README.

Testing

• cargo test -p buzz-core -p buzz-sdk huddle — 9 passed, 0 failed (5 core reader + 4 SDK builder, including a round-trip from the SDK builder through the core reader).
• cargo check -p buzz-relay (SQLX_OFFLINE) — clean.
• cargo fmt — clean.
• Zero unsafe; no panics/unwraps on the new paths.

Follow-ups (the rest of #2513) — designed, not in this PR

1. Server-side per-speaker recording. Tap the audio room's frame fan-out, buffer Opus per speaker pubkey, and persist to the existing media store (crates/buzz-media, S3/MinIO/Blossom) on huddle end. Needs a memory/retention budget per room.
2. Transcription over per-speaker tracks. A pluggable/off-box transcriber runs per track; because segments are already attributed, output is "speaker → text → time range" with no speaker-separation. A multilingual model (e.g. Parakeet v3, see Huddle transcription is English-only: non-English speech (e.g. Korean) produces garbled transcripts #2478 / PR feat(huddle): make STT model selectable + add multilingual Parakeet v3 (#2478) #2520) makes this multilingual by construction.
3. Consent + retention. Recording is opt-in per huddle and announced via KIND_HUDDLE_GUIDELINES (48106) as a "this huddle is recorded" policy; a NIP-09 deletion path removes transcript events + their audio objects.

Splitting these keeps each PR reviewable; this one lands the protocol contract they all build on.

Refs #2513