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.cpp

@@ -15,6 +15,7 @@
 #include "Engine/World.h"
 #include "TimerManager.h"
 #include "Containers/Ticker.h"
+#include "HAL/IConsoleManager.h"
 #include "Kismet/GameplayStatics.h"
 #include "Misc/CommandLine.h"
 #include "Misc/Parse.h"
@@ -132,6 +133,8 @@ AGigLineCharacter::AGigLineCharacter()
 		TEXT("/Game/Input/IA_Jump.IA_Jump"));
 	static ConstructorHelpers::FObjectFinder<UInputAction> ResetF(
 		TEXT("/Game/Input/IA_Reset.IA_Reset"));
+	static ConstructorHelpers::FObjectFinder<UInputAction> DebugF(
+		TEXT("/Game/Input/IA_ToggleDebug.IA_ToggleDebug"));
 	static ConstructorHelpers::FObjectFinder<UInputAction> CamF(
 		TEXT("/Game/Input/IA_ToggleCamera.IA_ToggleCamera"));
 	static ConstructorHelpers::FObjectFinder<UInputAction> PauseF(
@@ -146,6 +149,7 @@ AGigLineCharacter::AGigLineCharacter()
 	JumpAction = JumpF.Object;
 	ResetAction = ResetF.Object;
 	ToggleCameraAction = CamF.Object;
+	ToggleDebugAction = DebugF.Object;
 	PauseAction = PauseF.Object;
 
 	if (!MappingContext || !MoveAction || !MoveBackAction || !StrafeAction || !StrafeLeftAction
@@ -257,6 +261,28 @@ void AGigLineCharacter::BeginPlay()
 	// with `-thirdperson` so a capture can show the walk clip mid-stride: an idle
 	// shot cannot prove the animation set, and a still cannot show a stride.
 	bWalkForever = FParse::Param(FCommandLine::Get(), TEXT("walk"));
+
+	// `-perf` starts the overlay on: the performance capture and any manual frame
+	// check should not depend on someone pressing F3 first. The built demo has
+	// neither switch, so the counter stays hidden for players.
+	if (FParse::Param(FCommandLine::Get(), TEXT("perf")) || FParse::Param(FCommandLine::Get(), TEXT("qaselftest")))
+	{
+		if (IConsoleVariable* Var = IConsoleManager::Get().FindConsoleVariable(TEXT("gigline.Debug")))
+		{
+			Var->Set(1, ECVF_SetByConsole);
+		}
+	}
+}
+
+void AGigLineCharacter::ToggleDebugOverlay()
+{
+	static IConsoleVariable* Var = IConsoleManager::Get().FindConsoleVariable(TEXT("gigline.Debug"));
+	if (Var)
+	{
+		const bool bNowOn = Var->GetInt() == 0;
+		Var->Set(bNowOn ? 1 : 0, ECVF_SetByConsole);
+		UE_LOG(LogTemp, Log, TEXT("GigLine: debug overlay %s"), bNowOn ? TEXT("on") : TEXT("off"));
+	}
 }
 
 void AGigLineCharacter::SetReviewWalk(bool bEnable)
@@ -658,6 +684,10 @@ void AGigLineCharacter::SetupPlayerInputComponent(UInputComponent* PlayerInputCo
 	{
 		Input->BindAction(ToggleCameraAction, ETriggerEvent::Started, this, &AGigLineCharacter::ToggleCamera);
 	}
+	if (ToggleDebugAction)
+	{
+		Input->BindAction(ToggleDebugAction, ETriggerEvent::Started, this, &AGigLineCharacter::ToggleDebugOverlay);
+	}
 	if (JumpAction)
 	{
 		Input->BindAction(JumpAction, ETriggerEvent::Started, this, &AGigLineCharacter::StartJump);