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>

bitchat/App/AppArchitecture.swift

@@ -356,6 +356,32 @@ final class LegacyConversationStore: ObservableObject {
         markRead(directConversationID(for: peerID, identityResolver: identityResolver))
     }
 
+    // MARK: Migration step 2 bridge entry points (DELETE IN STEP 5)
+    // Used only by `LegacyConversationStoreBridge` to mirror single
+    // conversations out of the new `ConversationStore` without the
+    // full-dictionary `synchronizePrivateChats` pass.
+
+    func replaceDirectMessages(
+        _ messages: [BitchatMessage],
+        for peerID: PeerID,
+        identityResolver: IdentityResolver
+    ) {
+        let handle = identityResolver.canonicalHandle(for: peerID, displayName: messages.last?.sender)
+        let conversationID = ConversationID.direct(handle)
+        directHandlesByConversation[conversationID] = handle
+        replaceMessages(messages, for: conversationID)
+    }
+
+    func markUnread(
+        peerID: PeerID,
+        identityResolver: IdentityResolver
+    ) {
+        let conversationID = directConversationID(for: peerID, identityResolver: identityResolver)
+        if !unreadConversations.contains(conversationID) {
+            unreadConversations.insert(conversationID)
+        }
+    }
+
     private func normalized(_ messages: [BitchatMessage]) -> [BitchatMessage] {
         var uniqueMessages: [String: BitchatMessage] = [:]