[REFACTOR] Graduate the @experimental single-consumer turn-lifecycle seams

[vutuanlinh2k2]

Context

Six seams in src/chat-routes/turn-routes.ts are marked @experimental Single-consumer (gtm, #200); shape may change: turnLock, contextGate, beforeTurn, onRawEvent, and the heartbeat seams. Until a second consumer validates them (or they are generalized), the vertical's contract is not stable.

Problem / Goal

Stabilize the seam shapes and drop the @experimental caveats.

Proposed Scope

• As tax/legal/creative migrations exercise these seams, fix any leaks of gtm assumptions.
• Remove the markers once each seam has ≥2 consumers or a deliberate generalized shape.

Acceptance Criteria

• Each of the six seams is either graduated (marker removed, shape documented) or deliberately deprecated.

Notes

Depends on the app-adoption issues in the consolidation epic. Related: #188.