Fix mouse look swallowing by the HUD

Move guard look handling to the _input stage, recapture the cursor on
window-focus return and when clicking back into gameplay, and make HUD controls
mouse-transparent. Add regression tests for yaw/pitch and HUD mouse filters.
Verified under Xvfb with real relative mouse motion. Headless 237/237, Python
94/94, movement smoke 60 fps.

src/test/tests/test_guard_input.gd

@@ -0,0 +1,38 @@
+extends RefCounted
+## Regression tests for the first-person mouse-look path and non-blocking HUD.
+
+const GameManager := preload("res://src/game/game_manager.gd")
+const GuardController := preload("res://src/game/guard_controller.gd")
+const Hud := preload("res://src/game/ui/hud.gd")
+
+func test_mouse_look_rotates_yaw_and_pitch() -> bool:
+	var root := Node3D.new()
+	var gm: Node = GameManager.new()
+	var guard: GuardController = gm.call("_build_guard")
+	guard._ready()
+	var yaw0 := guard.rotation.y
+	var pitch0 := guard.head.rotation.x
+	guard._apply_mouse_look(Vector2(100, 50))
+	var a := TestUtils.check(guard.rotation.y < yaw0 - 0.1, "horizontal motion yaws the guard")
+	var b := TestUtils.check(guard.head.rotation.x < pitch0 - 0.1, "vertical motion pitches the head")
+	guard._apply_mouse_look(Vector2(0, 100000))
+	var c := TestUtils.check(absf(rad_to_deg(guard.head.rotation.x)) <= 80.01,
+		"pitch stays clamped")
+	guard.free()
+	gm.free()
+	root.free()
+	return a and b and c
+
+func test_hud_does_not_capture_mouse_input() -> bool:
+	var hud: Hud = Hud.new()
+	var root := hud.get_child(0) as Control
+	var a := TestUtils.check(root != null and root.mouse_filter == Control.MOUSE_FILTER_IGNORE,
+		"full-screen HUD root ignores mouse input")
+	var labels := hud.find_children("*", "Label", true, false)
+	var blocking := 0
+	for node in labels:
+		if (node as Control).mouse_filter != Control.MOUSE_FILTER_IGNORE:
+			blocking += 1
+	var b := TestUtils.check(blocking == 0, "HUD labels do not swallow mouse motion")
+	hud.free()
+	return a and b