refactor(audit): rename Fan Stress Test → GPU Platform Stress Test

Update all user-facing strings in TUI and ActionResult title.
Internal identifiers (types, functions, file name) unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/app/app.go

@@ -505,7 +505,7 @@ func (a *App) RunFanStressTestResult(ctx context.Context, opts platform.FanStres
 	if err != nil && err != context.Canceled {
 		body += "\nERROR: " + err.Error()
 	}
-	return ActionResult{Title: "Fan Stress Test", Body: body}, err
+	return ActionResult{Title: "GPU Platform Stress Test", Body: body}, err
 }
 
 // formatFanStressResult formats the summary.txt from a fan-stress run, including