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/AppRuntime.swift

@@ -15,6 +15,11 @@ final class AppRuntime: ObservableObject {
     let chatViewModel: ChatViewModel
     let events = AppEventStream()
     let conversationStore: LegacyConversationStore
+    /// Single source of truth for conversation message state
+    /// (docs/CONVERSATION-STORE-DESIGN.md). The legacy store above keeps
+    /// feeding the feature models until step 5, mirrored from this one by
+    /// `LegacyConversationStoreBridge`.
+    let conversations: ConversationStore
     let peerIdentityStore: PeerIdentityStore
     let locationPresenceStore: LocationPresenceStore
     let publicChatModel: PublicChatModel
@@ -44,10 +49,12 @@ final class AppRuntime: ObservableObject {
         self.idBridge = idBridge
         let identityResolver = IdentityResolver()
         let conversationStore = LegacyConversationStore()
+        let conversations = ConversationStore()
         let peerIdentityStore = PeerIdentityStore()
         let locationPresenceStore = LocationPresenceStore()
         let locationManager = LocationChannelManager.shared
         self.conversationStore = conversationStore
+        self.conversations = conversations
         self.peerIdentityStore = peerIdentityStore
         self.locationPresenceStore = locationPresenceStore
         self.chatViewModel = ChatViewModel(
@@ -55,6 +62,7 @@ final class AppRuntime: ObservableObject {
             idBridge: idBridge,
             identityManager: SecureIdentityStateManager(keychain),
             conversationStore: conversationStore,
+            conversations: conversations,
             identityResolver: identityResolver,
             peerIdentityStore: peerIdentityStore,
             locationPresenceStore: locationPresenceStore,