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/ChatDeliveryCoordinator.swift

@@ -12,7 +12,11 @@ import Foundation
 /// coordinators off their `unowned let viewModel: ChatViewModel` back-refs.
 @MainActor
 protocol ChatDeliveryContext: AnyObject {
-    var messages: [BitchatMessage] { get set }
+    /// Get-only derived views of the `ConversationStore`. `BitchatMessage`
+    /// is a reference type, so the public-timeline status patch below writes
+    /// through the shared message objects; step 4 replaces this with
+    /// `setDeliveryStatus(for:in:)` store intents.
+    var messages: [BitchatMessage] { get }
     var privateChats: [PeerID: [BitchatMessage]] { get }
     var isStartupPhase: Bool { get }
     /// Applies a delivery status to a private message by ID (single-writer