Convert shared coordinator state to owner-side intent operations

nostrKeyMapping, sentGeoDeliveryAcks/sentReadReceipts dedupe,
isBatchingPublic, geo subscription lifecycle, and private-chat selection
hand-off now mutate through single intent operations on ChatViewModel,
with backing storage locked down via private(set) so the single-writer
property is compiler-enforced. Context protocols downgrade to read-only
access where reads remain. 8 new contract tests.

Known remaining writers outside the protocols: sentReadReceipts is
passed inout to PrivateChatManager.syncReadReceiptsForSentMessages and
un-marked by ChatTransportEventCoordinator on disconnect.

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

bitchatTests/ChatViewModelDeliveryStatusTests.swift

@@ -388,6 +388,12 @@ private final class MockChatDeliveryContext: ChatDeliveryContext {
     private(set) var notifyUIChangedCount = 0
     private(set) var markedDeliveredMessageIDs: [String] = []
 
+    func pruneSentReadReceipts(keeping validMessageIDs: Set<String>) -> Int {
+        let oldCount = sentReadReceipts.count
+        sentReadReceipts = sentReadReceipts.intersection(validMessageIDs)
+        return oldCount - sentReadReceipts.count
+    }
+
     func notifyUIChanged() {
         notifyUIChangedCount += 1
     }