40250 classic: increment 50 (Phase 1 W3) — §8.3 in-game help window

Implement the in-game Help window as a 1:1 migration of uihelp.py's
HelpWindow (single-page: ENABLE_HELP_MULTIPAGE = 0 -> LoadDialogSinglePage).

- project/ui/help_ui.gd (new): loads the real helpwindow.py uiscript via
  UiScript -> UiBuild; _relabel() resolves the <HELP_*>/<CLOSE> placeholders
  (produced when UiManager has no locale callback) from the project Locale
  table (en) with verbatim English fallbacks, covering the original script's
  two duplicate help_02 rows (right/middle camera). open/close/toggle/is_open;
  close_button -> close; ESC handled by the UiManager window stack.
- project/ui/system_menu_ui.gd: help_button changed from the placeholder toast
  to close() + open_help(); adds const HelpUI, an injectable help_ui member,
  _ensure_help() (lazy build, reuses the menu's ui/client/assets_root),
  open_help(), toggle_help(); setup() gains an optional 6th arg (old 5-arg
  callers unaffected).
- project/game_scene.gd: single elif in _unhandled_input — KEY_H without
  ctrl/meta -> system_menu_ui.toggle_help() (Ctrl+H stays free for
  /user_horse_ride).
- project/help_ui_test.gd (new): headless SceneTree test — helpwindow.py load,
  19 verbatim help rows + taskbar labels bound and non-placeholder,
  close_button text, rows() helper, close/open/toggle, ESC stack top,
  SystemMenuUI.open_help() lazy build.

Tests: help_ui_test, system_menu_ui_test, gamescene_test, ui_test all PASS
(plus system_option_ui_test, char_status_ui_test). No build/ in this worktree
so C++ ctest not run (no C++ change). Real-server and visual (.sub textures)
not verified. Docs: CLIENT-GAP.md W3 row + increment 50 bullet;
CLIENT-GAP-FIX.md §8.3 status/改动/测试/验收 + C.5 batch record.

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

project/ui/system_menu_ui.gd

@@ -7,7 +7,7 @@
 #   sm.toggle()      # ESC（无其它窗打开时）
 #
 # 按钮绑定逐字对照 uisystem.py：
-#   help_button          → OpenHelpWindow（暂无帮助窗 → 提示）
+#   help_button          → interface.OpenHelpWindow（help_ui.gd，§8.3）
 #   mall_button          → net.SendChatPacket("/in_game_mall")
 #   system_option_button → uiSystemOption.OptionDialog（system_option_ui）
 #   game_option_button   → uiGameOption.OptionDialog（game_option_ui）
@@ -17,6 +17,8 @@
 #   cancel_button / 标题栏 → 关闭
 extends Node
 
+const HelpUI = preload("res://ui/help_ui.gd")
+
 const LABELS := {
 	"help_button": "帮助", "mall_button": "商城", "system_option_button": "系统设置",
 	"game_option_button": "游戏设置", "change_button": "选择角色", "logout_button": "登出",
@@ -29,16 +31,18 @@ var assets_root := ""
 var uiscript_dir := ""
 var system_option_ui: Node
 var game_option_ui: Node
+var help_ui: Node                  # HelpUI（§8.3）—— 未注入时 _ensure_help() 懒建
 var toast: Callable = Callable()   # 可选：func(msg: String) 显示提示
 
 var _win: Dictionary = {}
 
 func setup(ui_manager: CanvasLayer, m2client: Node, assets := "",
-		sys_opt: Node = null, game_opt: Node = null) -> void:
+		sys_opt: Node = null, game_opt: Node = null, help_win: Node = null) -> void:
 	ui = ui_manager
 	client = m2client
 	system_option_ui = sys_opt
 	game_option_ui = game_opt
+	help_ui = help_win
 	assets_root = assets
 	if assets_root == "" and ui and "assets_root" in ui:
 		assets_root = ui.assets_root
@@ -88,11 +92,25 @@ func _btn(nm: String, cb: Callable) -> void:
 	if b is BaseButton:
 		b.pressed.connect(cb)
 
+# uisystem.py: self.interface.OpenHelpWindow()。未注入 help_ui 时懒建一个，
+# 复用本菜单的 ui / client / assets_root。
+func _ensure_help() -> Node:
+	if not is_instance_valid(help_ui):
+		help_ui = HelpUI.new()
+		add_child(help_ui)
+		help_ui.setup(ui, client, assets_root)
+	return help_ui
+
+func open_help() -> void:
+	_ensure_help().open()
+
+func toggle_help() -> void:
+	_ensure_help().toggle()
+
 func _wire() -> void:
 	_btn("help_button", func():
-		if toast.is_valid():
-			toast.call("帮助窗暂未实现")
-		)
+		close()
+		open_help())
 	_btn("mall_button", func():
 		if client and client.has_method("say"):
 			client.say(0, "/in_game_mall")   # net.SendChatPacket("/in_game_mall")