feat: step to adjacent decoded video frames

Sources/Core/player.h

@@ -17,7 +17,7 @@ typedef struct PlayerState {
     double duration;
     bool has_audio;
     bool has_video;
-    // True from accepted seek until all queued seeks on this timeline finish.
+    // True until all accepted seeks/frame steps on this timeline finish.
     bool seek_pending;
     // Display width / height, including sample aspect ratio; zero without video.
     double video_aspect_ratio;
@@ -32,6 +32,9 @@ bool player_open(Player *player, const char *path);
 bool player_play(Player *player);
 bool player_pause(Player *player);
 bool player_seek(Player *player, double seconds);
+// Direction must be -1 (previous) or +1 (next). Requires ready video media.
+// Pauses playback and selects the adjacent decoded presentation frame.
+bool player_step_frame(Player *player, int direction);
 bool player_close(Player *player);
 // Synchronous, thread-safe gain; survives open, seek, and close. Zero mutes.
 // Rejects null handles and non-finite/out-of-range values without changing gain.