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

client/tests/unit/test_system_dock.gd

@@ -0,0 +1,32 @@
+extends "res://addons/gut/test.gd"
+
+const SCENE := "res://scenes/shell/SystemDock.tscn"
+
+
+func _dock() -> SystemDock:
+	var d = load(SCENE).instantiate()
+	add_child_autofree(d)  # _ready() builds the buttons
+	return d
+
+
+func test_builds_all_six_screen_buttons():
+	var d := _dock()
+	assert_eq(d._screen_buttons.size(), 6)
+	assert_true(d._screen_buttons.has(&"spellbook"), "spellbook kept for mock fidelity")
+	assert_true(d._screen_buttons.has(&"party"))
+
+
+func test_pressing_a_button_emits_its_id():
+	var d := _dock()
+	watch_signals(d)
+	d._screen_buttons[&"inventory"].emit_signal("pressed")
+	assert_signal_emitted_with_parameters(d, "screen_requested", [&"inventory"])
+
+
+func test_handle_toggles_state():
+	var d := _dock()
+	var state := ShellState.seed()
+	d.setup(state)
+	assert_true(state.dock_open)
+	d._on_handle_pressed()
+	assert_false(state.dock_open, "handle flips the shared state")

client/tests/unit/test_system_dock.gd.uid

@@ -0,0 +1 @@
+uid://dllvm2kmia00h