Add FVPController class with working appendBuffer support

[Copilot]

appendBuffer in FVPControllerExtensions is fundamentally broken for its primary use case: _getId() reads textureId/playerId, which are only valid after initialize() completes — but appendBuffer must be called during initialization while prepare() is blocked waiting for data from a mdk: source.

Solution: FVPController class

Replaces the extension-based approach with a new FVPController extends VideoPlayerController that pre-creates the underlying MdkVideoPlayer in the constructor, keyed by its stable nativeHandle throughout the full lifecycle.

Platform changes (video_player_mdk.dart)

• _playersByHandle <int, MdkVideoPlayer> — tracks all FVP-managed players by nativeHandle from construction to disposal
• _promotedHandles <int> — O(1) set tracking which handles have been added to _players via create()
• _nextPlayerHandle — hint set by FVPController.initialize() so create() reuses the pre-created player instead of allocating a new one
• New methods: createPendingPlayer(), setNextPlayerHandle(), clearNextPlayerHandle(), discardPendingPlayer(), appendBufferByHandle()
• dispose() now cleans up _playersByHandle and _promotedHandles
• Removed the now-unused appendBuffer(playerId, ...) overload

Controller changes (controller.dart)

• Removes appendBuffer from FVPControllerExtensions
• Adds FVPController with .network(), .asset(), .file() constructors, appendBuffer(), and overridden initialize()/dispose()

Usage

final ctrl = FVPController.network('mdk:');
final initFuture = ctrl.initialize(); // don't await yet
ctrl.appendBuffer(chunk1);
ctrl.appendBuffer(chunk2, flags: -1); // -1 = end-of-stream
await initFuture; // completes once prepare() has enough data

appendBuffer works at all lifecycle stages — before, during, and after initialize().

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