Fix Scratch event flows disposal handling

[oco777]

Summary

• stop disposing Visual Scripting Flow instances in Scratch event units and let Flow.Coroutine manage lifecycle
• update VS_Scratch_Mapping documentation to describe the new Flow handling approach

Testing

• not run (not requested)

---

Codex Task

[chatgpt-codex-connector]

💡 Codex Review

FUnity/Runtime/Integrations/VisualScripting/Units/ScratchUnits/InputEventUnits.cs

Lines 153 to 158 in edb198f

	private IEnumerator RunEventCoroutine(Flow flow, EmptyEventArgs args)
	{
	if (!ShouldTrigger(flow, args))
	{
	yield break;
	}

Dispose Flow when key event does not trigger

RunEventCoroutine now returns immediately when ShouldTrigger is false, but the Flow allocated in TriggerWithThreadRegistration is no longer wrapped in a using or passed to Flow.StartCoroutine. Because this unit listens to EventHooks.Update, the monitored key will be unpressed most frames, so every frame allocates a Flow that is never disposed, steadily leaking memory/GC pressure during normal play. The prior using (flow) cleaned up these skipped events; some disposal is still needed for the non-trigger path.

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