Cut public message path over to ConversationStore; delete PublicTimelineStore

Mesh and geohash timelines are now store conversations. All public
mutation sites flow through store intents; PublicMessagePipeline keeps
its 80ms UI batching but commits batches via store appends with each
buffered entry carrying its destination conversation (a mid-batch
channel switch now flushes instead of dropping the buffer).
ChatViewModel.messages becomes a cached get-only view of the active
conversation, invalidated through the change subject. The mesh
late-insert threshold is consciously removed: it only ever ordered the
non-rendered messages copy, so strict timestamp insertion makes the
working set agree with rendered order. PublicTimelineStore and the
per-message full-array legacy sync are deleted; the coalescing bridge
mirrors public conversations for the remaining legacy readers.

pipeline.publicIngest: 6.6k -> 9.5k msg/s (+45%); private steady;
store.append 237k/s.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/CONVERSATION-STORE-DESIGN.md

@@ -1,8 +1,9 @@
 # Conversation Store: Single Source of Truth
 
-**Status:** Approved design, not yet implemented. Baselines recorded in
+**Status:** Steps 1–3 implemented (additive store, private cutover, public
+cutover; `PublicTimelineStore` deleted). Baselines recorded in
 `bitchatTests/Performance/PerformanceBaselineTests.swift` (`pipeline.privateIngest`,
-`pipeline.publicIngest`).
+`pipeline.publicIngest`, `store.append`).
 
 ---