Finish coordinator migration: zero ChatViewModel back-references remain

ChatPeerListCoordinator, ChatComposerCoordinator, ChatOutgoingCoordinator,
and GeoChannelCoordinator complete the migration; every coordinator now
depends on a narrow @MainActor context protocol. GeoChannelCoordinator's
three injected closures collapse into a weak context. New intent op
recordPublicActivity(forChannelKey:) keeps lastPublicActivityAt
single-writer. 15 new mock-context tests; flaky-poll deadline in the
gift-wrap dedup test raised for parallel load.

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

bitchat/ViewModels/ChatViewModelBootstrapper.swift

@@ -217,15 +217,7 @@ private extension ChatViewModelBootstrapper {
     func configureGeoChannels() {
         viewModel.geoChannelCoordinator = GeoChannelCoordinator(
             locationManager: viewModel.locationManager,
-            onChannelSwitch: { [weak viewModel] channel in
-                viewModel?.switchLocationChannel(to: channel)
-            },
-            beginSampling: { [weak viewModel] geohashes in
-                viewModel?.beginGeohashSampling(for: geohashes)
-            },
-            endSampling: { [weak viewModel] in
-                viewModel?.endGeohashSampling()
-            }
+            context: viewModel
         )
     }