feat(hud): F3 debug overlay — FPS, frame time, worst frame, state

A top-left readout for performance work: smoothed FPS and frame time, the worst
frame in a rolling two-second window, the active view, player speed, a frame
counter and the player's position in metres from the project origin.

- F3 toggles it, through the console variable gigline.Debug so a packaged build can
  be checked without a rebuild (IA_ToggleDebug).
- On automatically for -perf and -qaselftest, so a performance capture never depends
  on pressing a key first; off in the built demo, because a player should not see an
  FPS counter over Hollywood Boulevard.
- The first two seconds are a warm-up window excluded from the worst-frame figure.
  Without it the readout showed "worst 400 ms" for a healthy run: the startup shader
  and streaming hitches dominated the rolling maximum.

Measured cost on the self-test route at 1440p: 2.64 to 2.77 ms average, about 5 %,
still 6x the floor. The release-check number stays the overlay-off run; the
overlay-on series is committed beside it as
Docs/perf/0.2.0-walk-frametimes-overlay.csv.

Docs: controls (F3 row, overlay section, switch table), release checks (cost row),
architecture building block view (HUD and input list).

Tools/ue_create_input_assets.py

@@ -127,6 +127,10 @@ def build():
     ia_reset.set_editor_property("value_type", unreal.InputActionValueType.BOOLEAN)
     save(ia_reset)
 
+    ia_debug = fresh("IA_ToggleDebug", unreal.InputAction, unreal.InputAction_Factory)
+    ia_debug.set_editor_property("value_type", unreal.InputActionValueType.BOOLEAN)
+    save(ia_debug)
+
     ia_cam = fresh("IA_ToggleCamera", unreal.InputAction, unreal.InputAction_Factory)
     ia_cam.set_editor_property("value_type", unreal.InputActionValueType.BOOLEAN)
     save(ia_cam)
@@ -140,7 +144,7 @@ def build():
         (ia_move, "Gamepad_LeftY"), (ia_strafe, "Gamepad_LeftX"),
         (ia_look, "MouseX"), (ia_look, "MouseY"), (ia_look, "Gamepad_Right2D"),
         (ia_jump, "SpaceBar"), (ia_jump, "Gamepad_FaceButton_Bottom"),
-        (ia_reset, "R"), (ia_pause, "Escape"), (ia_cam, "V"),
+        (ia_reset, "R"), (ia_pause, "Escape"), (ia_cam, "V"), (ia_debug, "F3"),
     ]
 
     mappings = []