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/Performance/PerformanceBaselineTests.swift

@@ -655,6 +655,17 @@ private final class PerfDeliveryContext: ChatDeliveryContext {
         return oldCount - sentReadReceipts.count
     }
 
+    @discardableResult
+    func setPrivateDeliveryStatus(_ status: DeliveryStatus, forMessageID messageID: String, peerID: PeerID) -> Bool {
+        guard var chat = privateChats[peerID],
+              let index = chat.firstIndex(where: { $0.id == messageID }) else {
+            return false
+        }
+        chat[index].deliveryStatus = status
+        privateChats[peerID] = chat
+        return true
+    }
+
     /// 2000 public + `peerCount` x `messagesPerPeer` private messages with
     /// deterministic IDs and timestamps.
     static func makeCorpus(publicCount: Int, peerCount: Int, messagesPerPeer: Int) -> PerfDeliveryContext {