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>

bitchat/ViewModels/ChatViewModelBootstrapper.swift

@@ -131,7 +131,6 @@ private extension ChatViewModelBootstrapper {
         viewModel.meshService.startServices()
 
         viewModel.publicMessagePipeline.delegate = viewModel.publicConversationCoordinator
-        viewModel.publicMessagePipeline.updateActiveChannel(viewModel.activeChannel)
 
         DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) { [weak viewModel] in
             guard let viewModel,