PR A6b — Chat storage implementation (GATED: do not start before G1 is accepted)

STOP unless plans/g1-chat-stream-study.md exists and its recommendations have been accepted (check the plan's G1/A6b status lines). This PR implements the STUDY'S design, not the appendix recipes — the appendix's MessagesStore/QueueStore/accepted-plan-projection sections are evaluated inputs the study may have amended or rejected. If the study routed any item to the C3 host move instead, that item is out of scope here.

Read: plans/cleanup-state-machines.md (A6b section + rollout rules); plans/g1-chat-stream-study.md (the design of record for this PR); appendix recipes for chatMessagesByIdAtom, the queue pair, and planStateAtom (reader inventories and regression-test lists remain valid even where storage design changed). A6a has landed — status/error selectors and the plan_handoff facade exist; build on them.

Scope (as approved by the study — likely shape):

1. Messages: the study-approved owner for renderer message state, with the write funnel covering streaming writes, the version_preview replaceChatMessages facade (WITH the stream-active guard — nothing guards that write-write conflict today), and the component hydrators via a hydrate path. Land the store behind the existing write pattern first, flip readers second, delete chatMessagesByIdAtom last. This is the highest-regression-risk change in Phase A: full streaming E2E suite required.
2. Queue pair: one store for queued+paused (atomic dequeue reads paused synchronously before pop — splitting reintroduces read-skew). Preserve: memory-only per-item callbacks, item object identity (WeakMap cache keys), write-before-poke ordering, restore-as-paused hydration.
3. planStateAtom split: acceptedChatIds → plan_handoff projection (+useIsPlanAccepted surviving return-to-idle); plansByChatId → the study-approved owner (renamed UI atom or React Query); inject getPlanData into PlanHandoffDeps; delete the mark-plan-accepted command. The usePlan/usePlanEvents dual-source race moves intact with a code comment (out of scope).

Rules: same-PR consumer migration + atom deletion per unit; allowlist entries removed; behavior-preserving with enumerated deltas; transition changes only where the study sanctioned them (isolated commits).

Verify: typecheck, full unit tests, lint, full streaming E2E. Suites per recipes: chat_stream tests trio, useStreamChat.test.tsx, useQueuePersistence.test.ts, version_preview/commands.test.ts, PlanPanel coverage, hybrid harness. /deep-review. Branch cleanup-a6b-chat-storage; /pr-push; update plan status.