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/ConversationUIModel.swift

@@ -15,19 +15,19 @@ final class ConversationUIModel: ObservableObject {
 
     private let chatViewModel: ChatViewModel
     private let privateConversationModel: PrivateConversationModel
-    private let conversationStore: LegacyConversationStore
+    private let conversations: ConversationStore
     private var activeChannel: ChannelID
     private var cancellables = Set<AnyCancellable>()
 
     init(
         chatViewModel: ChatViewModel,
         privateConversationModel: PrivateConversationModel,
-        conversationStore: LegacyConversationStore
+        conversations: ConversationStore
     ) {
         self.chatViewModel = chatViewModel
         self.privateConversationModel = privateConversationModel
-        self.conversationStore = conversationStore
-        self.activeChannel = conversationStore.activeChannel
+        self.conversations = conversations
+        self.activeChannel = conversations.activeChannel
         self.currentNickname = chatViewModel.nickname
         self.isBatchingPublic = chatViewModel.isBatchingPublic
         self.showAutocomplete = chatViewModel.showAutocomplete
@@ -151,7 +151,7 @@ final class ConversationUIModel: ObservableObject {
             .receive(on: DispatchQueue.main)
             .assign(to: &$isBatchingPublic)
 
-        conversationStore.$activeChannel
+        conversations.$activeChannel
             .receive(on: DispatchQueue.main)
             .sink { [weak self] channel in
                 self?.activeChannel = channel