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>

bitchatTests/ChatViewModelRefactoringTests.swift

@@ -138,7 +138,7 @@ struct ChatViewModelRefactoringTests {
         // Wait for async processing with proper timeout
         let found = await TestHelpers.waitUntil(
             {
-                viewModel.timelineStore.messages(for: .mesh).contains(where: { $0.content == "Public Hi" })
+                viewModel.publicMessages(for: .mesh).contains(where: { $0.content == "Public Hi" })
             },
             timeout: TestConstants.defaultTimeout
         )