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/Performance/PerformanceBaselineTests.swift

@@ -389,9 +389,9 @@ final class PerformanceBaselineTests: XCTestCase {
     /// Baseline for the full public-message ingest cycle through a real
     /// `ChatViewModel`: `didReceivePublicMessage` (transport delegate entry,
     /// main-actor Task hop per message) → `handlePublicMessage` (rate limit,
-    /// `PublicTimelineStore` append, per-message full-array
-    /// `LegacyConversationStore` sync) → `PublicMessagePipeline` timer-batched
-    /// flush into `ChatViewModel.messages` → `PublicChatModel` mirror.
+    /// pipeline enqueue) → `PublicMessagePipeline` timer-batched flush into
+    /// the `ConversationStore` (derived `ChatViewModel.messages` view) →
+    /// coalesced `LegacyConversationStoreBridge` mirror → `PublicChatModel`.
     /// Measures until `messages` and the feature model reflect every message,
     /// so the pipeline's flush latency is part of the cycle. Senders are
     /// spread 4-per-peer to stay under the 5-token sender rate bucket.
@@ -567,13 +567,11 @@ private final class PerfNostrContext: ChatNostrContext {
     }
 
     var messages: [BitchatMessage] = []
-    func resetPublicMessagePipeline() {}
-    func updatePublicMessagePipelineChannel(_ channel: ChannelID) {}
+    func flushPublicMessagePipeline() {}
     func refreshVisibleMessages(from channel: ChannelID?) {}
     func addPublicSystemMessage(_ content: String) {}
     func drainPendingGeohashSystemMessages() -> [String] { [] }
     func appendGeohashMessageIfAbsent(_ message: BitchatMessage, toGeohash geohash: String) -> Bool { true }
-    func synchronizePublicConversationStore(forGeohash geohash: String) {}
 
     private(set) var handledPublicMessageCount = 0
     func handlePublicMessage(_ message: BitchatMessage) { handledPublicMessageCount += 1 }