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/audio.gd

@@ -19,6 +19,12 @@ var master_bgm := 0.6
 var master_sfx := 0.9
 var _queued_bgm := ""
 
+# §9.2 前后台音量存/恢复，字段名对齐参考 CSoundManager 的
+# m_fBackupMusicVolume / m_fBackupSoundVolume（REF/MilesLib/SoundManager.h:31）。
+var _backup_music_volume := -1.0
+var _backup_sound_volume := -1.0
+var _volume_saved := false
+
 func _ready() -> void:
 	if _queued_bgm != "":
 		var n := _queued_bgm
@@ -127,6 +133,32 @@ func _fade(pl: AudioStreamPlayer, to_db: float, dur: float, stop_after := false)
 	if stop_after:
 		tw.tween_callback(pl.stop)
 
+# 立即把 master_bgm 应用到当前正在播放的 BGM 声道（无淡入）。
+# 对标 CSoundManager::SetMusicVolume：改的是「当前音乐实例」的音量。
+func set_music_volume(v: float) -> void:
+	master_bgm = clampf(v, 0.0, 1.0)
+	var pl: AudioStreamPlayer = _bgm[_bgm_cur]
+	if pl and pl.playing:
+		pl.volume_db = linear_to_db(master_bgm) if master_bgm > 0.0 else -80.0
+
+# §9.2：进入后台前保存音量（CSoundManager::SaveVolume）。
+func save_volume() -> void:
+	_backup_music_volume = master_bgm
+	_backup_sound_volume = master_sfx
+	_volume_saved = true
+
+# §9.2：回到前台后恢复音量（CSoundManager::RestoreVolume）。
+# 不碰 _bgm_name / _bgm[]，同一首继续播，只把音量拉回来。
+func restore_volume() -> void:
+	if not _volume_saved:
+		return
+	master_bgm = _backup_music_volume
+	master_sfx = _backup_sound_volume
+	_volume_saved = false
+	var pl: AudioStreamPlayer = _bgm[_bgm_cur]
+	if pl and pl.playing:
+		pl.volume_db = linear_to_db(master_bgm) if master_bgm > 0.0 else -80.0
+
 # --- SFX -------------------------------------------------------------------
 
 func play_ui(name: String) -> void: