feat: project turn_id onto folded chat-history entries

[akandola]

What

Project each folded history frame's source ChatMessage.turnId onto the feed entry as turn_id. The fold historyToFeed currently drops it.

Why — completing an existing design

convergence/README.md (the resumable-stream design) states the intent already: turnId is "persisted on the turn's user+assistant ChatMessages … so a client resyncing across a turn boundary can match history to a live turn," with sync{resync:true} telling the client to "refetch history."

The data is already there — ChatMessage.turnId is persisted on both messages of a turn (packages/protocol/src/conversation.ts), and it's the same id the live stream stamps on wire frames (WireFrame.turnId). settleFromHistory already aligns a resync against history by turnId. The only gap is that the history fold never projected turnId onto the FeedFrame / ChatHistoryEntry it emits, so backfilled entries carried no per-turn identity. This PR closes that gap.

Consumer motivation (secondary)

For a client that ingests the live SSE stream and uses REST history as its recovery path: when a sleeping pod makes the resume cursor unserviceable, the server sends sync{resync:true} and the client refetches history. turn_id on the folded entries is what lets that refetched history be aligned to the turns already received live, instead of guessing.

Scope

• Strictly additive, client-SDK only — no server, wire, or protocol change. The server already returns turnId on every ChatMessage.
• Optional field — absent for pre-turn-id transcripts (covered by a test).
• FeedFrame (packages/sdk/src/modules/turns/history.ts) and its twin ChatHistoryEntry (ui/engine-client/src/types.ts) each gain turn_id?: string; historyToFeed threads it through every folded frame, mirroring how ts is already carried.

Tests

• @houston/sdk: 371/371 — unit tests assert turn_id is projected onto every frame of a turn (user bubble + assistant frames share one id) and is undefined for a pre-turn-id transcript.
• A contract test proves the server → REST → fold round-trip preserves turn_id, and that a whole settled turn folds under a single turn_id.
• houston-web: 218/218 (translate + history-cache unaffected). Full pnpm -r typecheck clean; Biome clean.

Naming

turn_id (snake) to match feed_type on the same object — consistency within the entry. Trivial to flip to turnId (matching the source field) if maintainers prefer.