fix(android): wait for a fresh PAG frame after FlutterView resume

[EverMPChat]

Summary

Fix the Android black frame that can appear when a FlutterView containing a PAG Texture is detached by a native tab and later resumed.

The branch is intentionally based on feature/fix_surface at ab78a6a4fcbfca99064bfc3fe498111f10023095 so it retains the newer Flutter SurfaceProducer lifecycle support.

Fixes #108

Changes

• track a frame generation for each composition and Surface lifecycle;
• emit onFrameReady only after a successful flush for the current generation;
• emit onFrameUnavailable when the Surface or lifecycle becomes invalid;
• keep the Android Texture mounted under defaultBuilder and wait one Flutter compositor frame before revealing it;
• add PAGViewState.pauseForLifecycle() for native-tab detach paths where no Surface-destroyed callback arrives;
• make Android start() flush the current progress before starting the animator;
• preserve existing non-Android Texture timing.

Why a lifecycle-specific pause

A normal animation pause must preserve existing display semantics. Native tab switching additionally needs to invalidate the previous ready state, because the underlying FlutterView can be detached without a matching SurfaceProducer.Callback.onSurfaceDestroyed callback.

Keeping this behavior in the shared adapter avoids requiring every Flutter page to add its own overlay or host-specific lifecycle handler.

Verification

• flutter test --coverage test/pag_view_frame_state_test.dart: 6/6 passed
• package analyze: no new errors; existing upstream/example warnings only
• example Android debug APK build: passed
• host Flutter PAG/card focused tests: 47/47 passed
• host Android lint and assemble: passed
• Android manual verification: repeated native tab switching no longer shows the black frame

The host app keeps libpag 4.5.85 through Gradle conflict resolution; this PR does not change the plugin dependency declaration.

[EverMPChat]

Superseded by #110, opened from the public EverMPChat fork. This pull request used a private head repository and is therefore not suitable for upstream review.