Cut views over to ConversationStore; delete legacy store, bridge, resolver

Feature models observe per-conversation objects directly: PublicChatModel
forwards the active Conversation's objectWillChange, PrivateInboxModel
republishes only for the selected peer's conversation - background
appends no longer invalidate foreground views. LegacyConversationStore,
the coalescing bridge, and IdentityResolver are deleted (resolver
canonicalization proved display-invisible: nothing enumerates direct
conversations, lookups are by exact peer ID, and raw keying is strictly
more robust - documented as a design deviation). Selection state moves
into the store. ChatViewModel.messages/privateChats survive as derived
read views for coordinators that genuinely need them; hot paths use a
new store-direct privateMessages(for:) witness.

Final numbers vs pre-migration baselines:
pipeline.privateIngest 9.7k -> 24.0k msg/s (2.5x)
pipeline.publicIngest  6.8k -> 13.7k msg/s (2.0x)
delivery updates       38k  -> 117-133k/s

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

bitchat/App/PeerListModel.swift

@@ -37,7 +37,7 @@ final class PeerListModel: ObservableObject {
     @Published private(set) var renderID = ""
 
     private let chatViewModel: ChatViewModel
-    private let conversationStore: LegacyConversationStore
+    private let conversations: ConversationStore
     private let locationChannelsModel: LocationChannelsModel
     private let peerIdentityStore: PeerIdentityStore
     private let locationPresenceStore: LocationPresenceStore
@@ -45,13 +45,13 @@ final class PeerListModel: ObservableObject {
 
     init(
         chatViewModel: ChatViewModel,
-        conversationStore: LegacyConversationStore,
+        conversations: ConversationStore,
         locationChannelsModel: LocationChannelsModel? = nil,
         peerIdentityStore: PeerIdentityStore? = nil,
         locationPresenceStore: LocationPresenceStore? = nil
     ) {
         self.chatViewModel = chatViewModel
-        self.conversationStore = conversationStore
+        self.conversations = conversations
         self.locationChannelsModel = locationChannelsModel ?? LocationChannelsModel()
         self.peerIdentityStore = peerIdentityStore ?? chatViewModel.peerIdentityStore
         self.locationPresenceStore = locationPresenceStore ?? chatViewModel.locationPresenceStore
@@ -122,7 +122,7 @@ final class PeerListModel: ObservableObject {
             }
             .store(in: &cancellables)
 
-        conversationStore.$unreadConversations
+        conversations.$unreadConversations
             .receive(on: DispatchQueue.main)
             .sink { [weak self] _ in
                 self?.refresh()