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.

project.godot

@@ -12,7 +12,7 @@ config_version=5
 
 config/name="Gig Line"
 config/description="Workplace security simulation about observation, access verification, and proportionate escalation."
-config/version="0.7.2"
+config/version="0.7.3"
 run/main_scene="res://scenes/main.tscn"
 config/features=PackedStringArray("4.3", "GL Compatibility")