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>

bitchatTests/CommandProcessorTests.swift

@@ -423,6 +423,12 @@ private final class MockCommandContextProvider: CommandContextProvider {
         clearCurrentPublicTimelineCallCount += 1
     }
 
+    private(set) var clearedPrivateChats: [PeerID] = []
+    func clearPrivateChat(_ peerID: PeerID) {
+        clearedPrivateChats.append(peerID)
+        privateChats[peerID] = []
+    }
+
     func sendPublicRaw(_ content: String) {
         sentPublicRawMessages.append(content)
     }