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).

Game/GigLine/Source/GigLine/GigLineCharacter.h

@@ -42,6 +42,9 @@ protected:
 	void TogglePause();
 
 public:
+	/** True while the third-person rig is the active view (for the debug overlay). */
+	bool IsThirdPerson() const { return bThirdPerson; }
+
 	/** Review harness: hold or release forward input from the screenshot actor. */
 	void SetReviewWalk(bool bEnable);
 
@@ -172,6 +175,13 @@ protected:
 	/** Applies the `-walk` forward input when the flag is set and the pawn is grounded. */
 	void WalkForever();
 
+	/** Debug-overlay action (F3). */
+	UPROPERTY(EditDefaultsOnly, BlueprintReadOnly, Category = "Input")
+	TObjectPtr<UInputAction> ToggleDebugAction;
+
+	/** Flips the gigline.Debug console variable that the HUD reads. */
+	void ToggleDebugOverlay();
+
 	/** Toggle-camera action (V). */
 	UPROPERTY(EditDefaultsOnly, BlueprintReadOnly, Category = "Input")
 	TObjectPtr<UInputAction> ToggleCameraAction;