feat: adopt IINA-style playback window

Sources/Core/player.h

@@ -21,7 +21,7 @@ typedef struct PlayerState {
     char error[512];
 } PlayerState;
 
-// Commands are asynchronous. False means invalid input or a full command queue.
+// 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);
 bool player_open(Player *player, const char *path);
@@ -29,6 +29,11 @@ bool player_play(Player *player);
 bool player_pause(Player *player);
 bool player_seek(Player *player, double seconds);
 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.
+bool player_set_volume(Player *player, float normalized0to1);
+// Returns the target gain immediately; zero for a null handle.
+float player_get_volume(Player *player);
 PlayerState player_get_state(Player *player);
 // Joins the worker and stops audio; never invoke from an audio callback.
 void player_destroy(Player *player);