feat(runtime): add canonical turn dispatch boundary

[Neonforge98]

Problem

Frontend turn producers independently combined lifecycle reservation, optimistic status, transport dispatch, idempotency, and terminal waiting. That made IPC response loss look like rejection even when the backend had already accepted the exact turn, let WorkItem composer intent X lose attribution when execution became WorkItemRun Y, and allowed a delayed terminal to be confused with a newer generation.

Solution

• Add one TurnDispatchService that synchronously reserves intent plus generation, sends through SessionService, reconciles durable receipts, and exposes exact-generation finality waits.
• Return typed acknowledgements from native and CLI adapters and share one Zod turn-intent status contract across both CLI receipts and agent_turn_intent_status.
• Alias backend-selected effective identities to the original frontend generation without overwriting conflicts.
• Retain bounded exact-generation terminals while preventing old terminals from changing a newer turn phase.
• Keep active intent mappings out of the recent-history LRU and retire them on terminal, dead-man, or session cleanup.
• Preserve rollout compatibility for legacy producers: backend-minted unknown intents are accepted only when the session has no active canonical mapping.

Potential risks

• This is a stacked PR and requires fix(runtime): make turn acceptance retry-safe #940; its base is codex/durable-turn-receipts so the typed backend receipt/status command is present first.
• Effective WorkItemRun and ambiguous response-loss receipts start a bounded-backoff local status monitor. It stops on exact terminal, generation supersession, session cleanup, or test reset; queued/running execution is intentionally not failed by elapsed time alone.
• Existing producers are not migrated in this PR. Their behavior remains compatible, and migration is isolated in the next PR.
• No Team Chat payload, Work Item UI, conversation plane, continuation store, cloud API, or database schema changes are included.

Verification

• 100 focused Vitest cases passed across TurnDispatchService, exact lifecycle, intent aliases, native receipt parsing/terminal attribution, CLI transport receipts, and shared schemas.
• pnpm typecheck
• ESLint and Prettier on all 21 changed files
• Focused madge traversal: 973 files, no circular dependency
• Normal pre-commit lint-staged, TypeScript, and commit-stats hooks passed

Architecture audit: SessionService remains the transport/context owner; TurnDispatchService owns only intent reservation, receipt reconciliation, and finality. UI transcript writes and conversation-plane projection remain outside it.

[Neonforge98]

Superseded by the final two-PR design: #844 removes the Work Item-specific remote continuation bridge, and stacked #939 keeps the reusable audience routing plus sender-local provider-native continuation. This dispatcher/store/handoff stack is intentionally retired.