40250 classic: parallel-dev checkpoint through increment 49 (W0 interface freeze)

Captures the uncommitted parallel-development work (increments 46-49) on the
40250 classic client port, folded into docs/CLIENT-GAP.md + CLIENT-GAP-FIX.md.

Increment 49 (W0 interface-freeze batch, Phase 1 prerequisite) lands the
cross-workflow shared-file scaffolding so the 4 Phase 1 workflows can run in
isolated worktrees without contention:

- entity_store.{h,cpp}: Entity gains empire/affect_flags/owner_vid/state_flags;
  new mut_spawn_full() (single-shot two-packet merge, §2.3), mut_ownership()
  (§2.5), mut_map_bgm() + take_bgm_dirty()/bgm_name()/bgm_volume() (§9.1),
  drain_dirty() (§2.1/§2.5 bare-field-update queue); mut_char_info() now stores
  empire; reset_for_map_change() clears m_dirty.
- m2_client.cpp: new bgm_changed(name, volume) signal; classic + m2dev pump
  loops drain drain_dirty() -> entity_info and take_bgm_dirty() -> bgm_changed;
  entity_dict() exposes the 4 new keys.
- classic/classic_parser.{h,cpp}: GC_MAIN_CHARACTER3_BGM /
  GC_MAIN_CHARACTER4_BGM_VOL route bgm_name/bgm_volume to mut_map_bgm() (was
  discarded); new m_pending_actor staging map (W1 fills §2.2 merge logic).
- project/bgm_director.gd (new) + game_scene.gd: BGM consumption split out of
  net_world.gd so W2/W4 don't collide; §8.6/§4.8 keybind convergence (digits
  1-4 -> quickslots 0-3, F1-F4 -> quickslots 4-7, Ctrl+1..9 -> _emote()).

Tests: cmake --build build clean; ctest 16/16; 15 GDScript regressions green
(netbridge, gamescene, netplay, p9, p10, p2b, p8, system_menu_ui, skill,
combat_fx, skill_fx, player_motion, char_status_ui, chat, inventory), no skips.

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

project/bgm_director.gd

@@ -0,0 +1,39 @@
+# BgmDirector —— 把 M2Client.bgm_changed 信号翻译成 Audio 的 BGM 播放。
+#
+# 存在的理由：§9.1 的地图 BGM 消费点，从 net_world.gd 里拆出来，避免 W2（可见性）
+# 和 W4（音频）同时改 net_world.gd。真正的「地图 → 曲目」解析和音量应用由 W4 在
+# 这里补全（见 docs/CLIENT-GAP-FIX.md §9.1）。
+#
+# 用法：
+#   var bd := preload("res://bgm_director.gd").new()
+#   add_child(bd)
+#   bd.bind(client, audio)
+extends Node
+
+var _client: Node
+var _audio: Node
+
+# 服务器最近一次下发的 BGM（曲目 id 或路径）和音量（<0 表示未指定）。
+var current_track := ""
+var current_volume := -1.0
+
+func bind(client: Node, audio: Node) -> void:
+	_client = client
+	_audio = audio
+	if _client and _client.has_signal("bgm_changed"):
+		_client.bgm_changed.connect(_on_bgm_changed)
+
+func _on_bgm_changed(name: String, volume: float) -> void:
+	current_track = name
+	if volume >= 0.0:
+		current_volume = volume
+		if _audio and "master_bgm" in _audio:
+			_audio.master_bgm = clampf(volume, 0.0, 1.0)
+	if _audio == null:
+		return
+	if name == "":
+		if _audio.has_method("stop_bgm"):
+			_audio.stop_bgm()
+		return
+	if _audio.has_method("play_bgm"):
+		_audio.play_bgm(name)