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 Previously unpublished state and injected action hooks are @ObservationIgnored.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/Views/GlobeInputModeButton.swift

@@ -12,7 +12,7 @@ import SwiftUI
 import UIKit
 
 struct GlobeInputModeButton: UIViewRepresentable {
-    @ObservedObject var state: KeyboardState
+    var state: KeyboardState
     @Binding var isPressed: Bool
 
     func makeUIView(context: Context) -> GlobeInputModeButtonView {
@@ -41,7 +41,7 @@ struct GlobeInputModeButton: UIViewRepresentable {
 struct SystemGlobeKey: View {
     @Environment(\.colorScheme) private var colorScheme
     @Environment(\.themePalette) private var palette
-    @ObservedObject var state: KeyboardState
+    var state: KeyboardState
     @State private var isPressed = false
 
     var width: CGFloat = 44