feat(media-store): add fullscreen button component state definition

Add reusable state definition for fullscreen button components:
- FullscreenButtonState interface with fullscreen boolean
- Explicit request methods: requestEnterFullscreen, requestExitFullscreen
- Follows established pattern similar to play/pause and mute/unmute
- Platform-agnostic state logic shared between HTML and React

State definition provides consistent behavior across all platforms
while enabling explicit control over fullscreen enter/exit actions.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

packages/core/media-store/src/component-state-definitions/fullscreen-button.ts

@@ -0,0 +1,36 @@
+export interface FullscreenButtonState {
+  fullscreen: boolean;
+}
+
+export interface FullscreenButtonMethods {
+  requestEnterFullscreen: () => void;
+  requestExitFullscreen: () => void;
+}
+
+export interface FullscreenButtonStateDefinition {
+  keys: string[];
+  stateTransform: (rawState: any) => FullscreenButtonState;
+  createRequestMethods: (
+    dispatch: (action: { type: string; detail?: any }) => void,
+  ) => FullscreenButtonMethods;
+}
+
+/**
+ * FullscreenButton state definition
+ * Defines the core state logic that can be shared between implementations
+ */
+export const fullscreenButtonStateDefinition: FullscreenButtonStateDefinition =
+  {
+    keys: ['fullscreen'],
+
+    stateTransform: (rawState: any): FullscreenButtonState => ({
+      fullscreen: rawState.fullscreen ?? false,
+    }),
+
+    createRequestMethods: (dispatch): FullscreenButtonMethods => ({
+      requestEnterFullscreen: () =>
+        dispatch({ type: 'fullscreenrequest', detail: true }),
+      requestExitFullscreen: () =>
+        dispatch({ type: 'fullscreenrequest', detail: false }),
+    }),
+  };

packages/core/media-store/src/index.ts

@@ -6,4 +6,5 @@ export * from './state-mediators/temporal';
 export * from './component-state-definitions/play-button';
 export * from './component-state-definitions/mute-button';
 export * from './component-state-definitions/volume-range';
-export * from './component-state-definitions/time-range';
+export * from './component-state-definitions/time-range';
+export * from './component-state-definitions/fullscreen-button';