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/AppArchitectureTests.swift

@@ -631,7 +631,7 @@ struct AppArchitectureTests {
         transport.reachablePeers.insert(otherPeerID)
         viewModel.nickname = "builder"
         viewModel.verifiedFingerprints.insert(verifiedFingerprint)
-        viewModel.unreadPrivateMessages = Set([otherPeerID])
+        viewModel.markPrivateChatUnread(otherPeerID)
         transport.updatePeerSnapshots([
             makeArchitectureSnapshot(
                 peerID: myPeerID,