40250 classic: increment 50 (W4) — map BGM selection + background volume save/restore

§9.1 map BGM selection (bgm_director.gd):
- server bgm_changed(name, volume): non-empty name plays it as field music;
  empty name falls back to musicInfo.fieldMusic (METIN2THEMA = "M2BG.mp3")
- volume < 0 (server unspecified, GC_MAIN_CHARACTER3_BGM) -> default
  1/5*0.1 = 0.02; otherwise server volume; volume applied before play_bgm()
  fade-in (matches game.py:190 SetMusicVolume/SetSoundVolume then FadeInMusic)
- connect M2Client.world_reset -> re-run selection on map change
- new musicinfo.gd: METIN2THEMA/loginMusic/createMusic/selectMusic/fieldMusic
  constants + save/load_last_play_field_music -> user://BGM/lastplay.inf

§9.2 background/foreground volume save-restore:
- audio.gd: save_volume()/restore_volume() (backup fields echo reference
  m_fBackupMusicVolume / m_fBackupSoundVolume) + set_music_volume()
- app_lifecycle.gd: _enter_background() uses save_volume() + mute instead of
  stop_bgm(); _exit_background() calls restore_volume(); _bgm_name never cleared

§9.3 lifecycle ownership: DOC ONLY — design confirmed, deferred to Phase 2 / W0
(AppFlow sole lifecycle owner; M2Client drops NOTIFICATION_* keeps suspend/resume).
game_scene.gd / app_flow.gd / m2_client.* left untouched per workflow constraints.

Tests: bgm_test.gd (new) PASS; gamescene_test / netplay_test / p9_test green.
Docs: CLIENT-GAP.md increment 50 bullet + W4 progress row; CLIENT-GAP-FIX.md
§9.1 §9.2 状态/改动/测试/验收, §9.3 deferred note, C.5 batch record.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014yvAPqPivoY7vmBbzgqK4W

project/app_lifecycle.gd

@@ -64,7 +64,13 @@ func _enter_background() -> void:
 	if background_max_fps > 0:
 		_saved_max_fps = Engine.max_fps
 		Engine.max_fps = background_max_fps
-	if _bound_audio and _bound_audio.has_method("stop_bgm"):
+	# §9.2：不停 BGM，改为「保存音量 + 静音」（对标 CSoundManager::SaveVolume 后
+	# SetMusicVolume(0)）。_bgm_name 不清空，回前台恢复音量即可同一首继续播。
+	if _bound_audio and _bound_audio.has_method("save_volume"):
+		_bound_audio.save_volume()
+		if _bound_audio.has_method("set_music_volume"):
+			_bound_audio.set_music_volume(0.0)
+	elif _bound_audio and _bound_audio.has_method("stop_bgm"):
 		_bound_audio.stop_bgm()
 	if _bound_client and _bound_client.has_method("suspend"):
 		_bound_client.suspend()
@@ -80,6 +86,9 @@ func _exit_background() -> void:
 		get_tree().paused = false
 	if background_max_fps > 0:
 		Engine.max_fps = _saved_max_fps
+	# §9.2：还原后台前保存的音量。
+	if _bound_audio and _bound_audio.has_method("restore_volume"):
+		_bound_audio.restore_volume()
 	if _bound_client and _bound_client.has_method("resume"):
 		_bound_client.resume()
 	resumed.emit()