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/TestUtilities/TestHelpers.swift

@@ -153,6 +153,24 @@ extension ChatViewModel {
         }
     }
 
+    /// Test-only replacement for the deleted `messages` setter: seeds a
+    /// public channel's conversation through the single-writer
+    /// `ConversationStore` intents (upsert keeps re-seeding with updated
+    /// copies working the way the old array assignment did).
+    @MainActor
+    func seedPublicMessages(_ messages: [BitchatMessage], for channel: ChannelID = .mesh) {
+        for message in messages {
+            conversations.upsertByID(message, in: ConversationID(channelID: channel))
+        }
+    }
+
+    /// Test-only replacement for `messages.removeAll()`: empties a public
+    /// channel's conversation.
+    @MainActor
+    func clearPublicMessages(for channel: ChannelID = .mesh) {
+        conversations.clear(ConversationID(channelID: channel))
+    }
+
     /// Test-only: drops every private chat and unread flag.
     @MainActor
     func clearAllPrivateChats() {