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>

bitchatTests/ChatViewModelTests.swift

@@ -553,10 +553,7 @@ struct ChatViewModelNoisePayloadTests {
         }, timeout: TestConstants.defaultTimeout)
 
         let conversationStoreUpdated = await TestHelpers.waitUntil({
-            let messages = viewModel.conversationStore.directMessages(
-                for: peerID,
-                identityResolver: viewModel.identityResolver
-            )
+            let messages = viewModel.conversations.conversationsByID[.directPeer(peerID)]?.messages ?? []
             guard let status = messages.first?.deliveryStatus else { return false }
             if case .read = status {
                 return true