feat: stream YouTube video with coordinated audio and video inputs

Resolve public YouTube URLs through external yt-dlp and play selected streams in the existing native window.

Independent bounded network readers keep one authored timeline. Successful seeks clear FFmpeg's sticky aborted-read AVIO error; manifest seekability uses native format flags rather than a top-level I/O handle.

Verified real YouTube playback, seeking, frame stepping, compact mode, cancellation, and the complete native regression checks.

Sources/Core/player.h

@@ -17,6 +17,10 @@ typedef struct PlayerState {
     double duration;
     bool has_audio;
     bool has_video;
+    // False for live/non-seekable inputs; reflects both inputs for separate streams.
+    bool can_seek;
+    // Network playback is waiting for media; the authoritative clock remains stalled.
+    bool buffering;
     // True until all accepted seeks/frame steps on this timeline finish.
     bool seek_pending;
     // Display width / height, including sample aspect ratio; zero without video.
@@ -28,7 +32,19 @@ typedef struct PlayerState {
 // Playback commands are asynchronous. False means invalid input or a full command queue.
 // Paths are copied before returning. All other calls require a live handle.
 Player *player_create(void);
+typedef struct PlayerNetworkSource {
+    // Primary video or combined audio/video HTTP(S) URL; audio_url is optional.
+    const char *video_url;
+    const char *audio_url;
+    // Optional HTTP headers encoded as "Name: value\r\n"; copied with the URLs.
+    const char *video_headers;
+    const char *audio_headers;
+    double duration;
+    bool is_live;
+} PlayerNetworkSource;
 bool player_open(Player *player, const char *path);
+// Streams directly; never downloads or muxes the entire media file before opening.
+bool player_open_network(Player *player, const PlayerNetworkSource *source);
 bool player_play(Player *player);
 bool player_pause(Player *player);
 bool player_seek(Player *player, double seconds);