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/charselect.gd

@@ -1,6 +1,6 @@
 # 选人界面 —— 3 个旋转台角色 + 九宫格信息面板 + 选择/创建按钮。
 # 数据来自 M2Client.char_list（联机）或 --mock 合成列表（离线开发）。
-# 选中 -> M2Client.select_character(index) -> entered_game。
+# 选中 -> M2Client.enter_game(index) / DirectEnter -> entered_game。
 # 用法（离线预览）：godot --path project --script charselect.gd --mock --quit-after 120
 extends SceneTree
 
@@ -209,8 +209,13 @@ func _enter() -> void:
 		_set_status("空槽不能进入（创建角色未实现）")
 		return
 	if _client:
-		_client.call("select_character", int(s.get("index", _sel)))
-		_set_status("select_character(%d) sent" % int(s.get("index", _sel)))
+		var idx := int(s.get("index", _sel))
+		if _client.has_method("enter_game"):
+			_client.call("enter_game", idx)
+			_set_status("enter_game(%d) sent" % idx)
+		else:
+			_client.call("select_character", idx)
+			_set_status("select_character(%d) sent" % idx)
 	else:
 		_set_status("[mock] 进入游戏：%s" % s.get("name"))