Cut private message path over to ConversationStore

All private-message mutations now flow through store intents:
coordinators, PrivateChatManager (its @Published dicts deleted - now
read-only views over the store), outbound sends, delivery status, and
chat migration. The O(1) store dedup replaces the full-scan duplicate
check; insertion order is maintained by the store so sanitizeChat's
re-sort is a documented no-op. Both bootstrapper Combine bridges and
the Task.yield store synchronization are deleted.

ChatViewModel.privateChats/unreadPrivateMessages become get-only derived
views (measured: naive rebuild equals a change-invalidated cache within
noise, so the simpler form stays). Feature models still read the legacy
store, fed by a coalescing LegacyConversationStoreBridge (one mirror
per burst, marked for step-5 deletion).

pipeline.privateIngest: 9.6k -> 14.7k msg/s (+53%).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bitchatTests/TestUtilities/TestHelpers.swift

@@ -138,6 +138,28 @@ enum TestError: Error {
     case testFailure(String)
 }
 
+// MARK: - Private chat seeding (ConversationStore migration)
+
+extension ChatViewModel {
+    /// Test-only replacement for the deleted `privateChats` setter: seeds a
+    /// peer's chat through the single-writer `ConversationStore` intents
+    /// (upsert keeps re-seeding with updated copies working the way the old
+    /// dictionary assignment did).
+    @MainActor
+    func seedPrivateChat(_ messages: [BitchatMessage], for peerID: PeerID) {
+        _ = conversations.conversation(for: .directPeer(peerID))
+        for message in messages {
+            conversations.upsertByID(message, in: .directPeer(peerID))
+        }
+    }
+
+    /// Test-only: drops every private chat and unread flag.
+    @MainActor
+    func clearAllPrivateChats() {
+        conversations.removeAllDirectConversations()
+    }
+}
+
 func sleep(_ seconds: TimeInterval) async throws {
     try await Task.sleep(nanoseconds: UInt64(seconds * 1_000_000_000))
 }