feat: adopt IINA-style playback window

Sources/Core/output.hpp

@@ -18,6 +18,9 @@ public:
     bool configure_audio(std::string &error);
     bool start(std::string &error);
     void pause();
+    // Atomic target gain; callback slews to it within 240 frames (5 ms).
+    void set_volume(float normalized0to1) noexcept;
+    float volume() const noexcept;
     // Called with audio stopped. Clears audio and video from the old timeline.
     void reset();
     std::size_t write(const float *samples, std::size_t frames, double pts);

Sources/Core/player.cpp

@@ -748,6 +748,16 @@ bool player_seek(Player *player, double seconds) {
 bool player_close(Player *player) {
     return submit(player, [=] { return player->enqueue({Action::Close, {}}); });
 }
+bool player_set_volume(Player *player, float normalized0to1) {
+    if (!std::isfinite(normalized0to1) || normalized0to1 < 0 || normalized0to1 > 1) return false;
+    return submit(player, [=] {
+        player->output.set_volume(normalized0to1);
+        return true;
+    });
+}
+float player_get_volume(Player *player) {
+    return player ? player->output.volume() : 0;
+}
 PlayerState player_get_state(Player *player) {
     try {
         if (player) {

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);