perf(keyboard): track KeyboardState per property with @Observable

- KeyboardState moves from ObservableObject/@Published to @Observable, so a view re-renders only for the properties it reads. Previously unpublished state and injected action hooks are @ObservationIgnored.
- KeyboardViewController replaces the $phase/$surface Combine sinks with Observations loops tied to the same cancellables.
- AIKeyboardView reads the 200 ms mic level and live ASR partials in small child views (AssistantWaveform, LiveTranscriptReader), so those ticks no longer rebuild the whole assistant surface.
- Tests observe changes with withObservationTracking instead of Combine publishers.

OSGKeyboardExt/Typing/KeyboardSurfaceRoot.swift

@@ -10,7 +10,7 @@ struct KeyboardSurfaceRoot: View {
     @Environment(\.colorScheme) private var colorScheme
     @Namespace private var keyboardTabSelectionNamespace
 
-    @ObservedObject var state: KeyboardState
+    var state: KeyboardState
     @ObservedObject var typing: TypingSessionController
 
     var onInsert: (String) -> Void

OSGKeyboardExt/Typing/TypingRootView.swift

@@ -38,7 +38,7 @@ enum TypingLayoutMetrics {
 
 struct TypingRootView: View {
     @Environment(\.colorScheme) private var colorScheme
-    @ObservedObject var state: KeyboardState
+    var state: KeyboardState
     @ObservedObject var typing: TypingSessionController
 
     var onInsert: (String) -> Void