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/p10_test.gd

@@ -119,7 +119,10 @@ func _run() -> void:
 	_ck(picked and fc2.selects == [0], "flow: pick char -> select_character(0)")
 	fc2.entered_game.emit()
 	_ck(af.state() == AppFlow.GAME, "flow: entered_game -> GAME")
-	# 在 SELECT 时断线 -> 回 LOGIN
+	# 在 GAME 时断线 -> 回 LOGIN（ClientVS22 SetLoginPhase 语义）
+	fc2.disconnected.emit("game dropped")
+	_ck(af.state() == AppFlow.LOGIN, "flow: disconnect in GAME -> LOGIN")
+	# 再次收到角色列表时允许回到 SELECT
 	fc2.char_list.emit([{"index": 0, "name": "Hero", "level": 42}])
 	_ck(af.state() == AppFlow.SELECT, "flow: back to SELECT")
 	fc2.disconnected.emit("dropped")