feat(shell): SystemDock slide-in with inert screen-routing seam

client/scripts/ui/shell/system_dock.gd

@@ -0,0 +1,68 @@
+class_name SystemDock
+extends Control
+## The slide-in system dock (mock 2a): a gold handle toggles a column of screen
+## buttons in/out. §2: state (owns the open toggle via ShellState). The buttons
+## are the routing seam — they emit screen_requested; the screens they open land
+## in later milestones, so the signal is currently a no-op for the shell.
+
+signal screen_requested(id: StringName)
+
+const SCREENS := [
+	[&"inventory", "Inventory", "I"],
+	[&"character", "Character", "C"],
+	[&"quest_log", "Quest Log", "J"],
+	[&"spellbook", "Spellbook", "K"],
+	[&"world_map", "World Map", "M"],
+	[&"party", "Party", "P"],
+]
+const SLIDE_SECONDS := 0.32
+const HIDDEN_SHIFT := 230.0
+
+var _screen_buttons: Dictionary = {}
+var _state: ShellState
+var _menu: VBoxContainer
+var _handle: Button
+
+
+func _ready() -> void:
+	_menu = VBoxContainer.new()
+	_menu.add_theme_constant_override("separation", 10)
+	add_child(_menu)
+	for entry in SCREENS:
+		var id: StringName = entry[0]
+		var button := Button.new()
+		button.text = "%s   %s" % [entry[2], entry[1]]
+		button.theme_type_variation = ThemeKeys.DARK_PANEL
+		button.pressed.connect(func(): screen_requested.emit(id))
+		_menu.add_child(button)
+		_screen_buttons[id] = button
+
+	_handle = Button.new()
+	_handle.text = "›"
+	_handle.theme_type_variation = ThemeKeys.TAB_ACTIVE
+	_handle.pressed.connect(_on_handle_pressed)
+	add_child(_handle)
+
+
+func setup(state: ShellState) -> void:
+	_state = state
+	_apply_open(_state.dock_open)
+
+
+func _on_handle_pressed() -> void:
+	if _state == null:
+		return
+	_apply_open(_state.toggle_dock())
+
+
+func _apply_open(open: bool) -> void:
+	_handle.text = "›" if open else "‹"
+	var target := Vector2.ZERO if open else Vector2(HIDDEN_SHIFT, 0.0)
+	var alpha := 1.0 if open else 0.0
+	if not is_inside_tree():
+		_menu.position = target
+		_menu.modulate.a = alpha
+		return
+	var tween := create_tween().set_parallel(true)
+	tween.tween_property(_menu, "position", target, SLIDE_SECONDS)
+	tween.tween_property(_menu, "modulate:a", alpha, SLIDE_SECONDS)

client/scripts/ui/shell/system_dock.gd.uid

@@ -0,0 +1 @@
+uid://dvbg24rvye3qw