Support Scratch `event_whengreaterthan` via generic `onCondition` trigger

[go-wyvern]

Background

• The current event system supports triggers like onStart, onKey, onMsg, and onTimer, but does not support Scratch event_whengreaterthan (when value > threshold).
• This causes behavior gaps when importing Scratch projects.
• Reference project: https://goplus-builder.qiniu.io/project/sb2xbp/970974930

Goal

• Add a generic condition-based event trigger:

onCondition(condition, () => {
  // ...
})

• Use this as the runtime mapping target for Scratch event_whengreaterthan.

Requirements

1. New Event API

• Add a public condition trigger registration API (suggested name: OnCondition / onCondition).
• Accept a boolean condition function/expression and a callback.

2. Trigger Semantics (critical)

• Use edge-trigger behavior: fire once on false -> true.
• Do not fire repeatedly every frame while condition remains true.
• Allow retrigger only after condition returns to false and becomes true again.
• This matches expected Scratch behavior and avoids callback storms.

3. Scratch Mapping

• Map event_whengreaterthan(option, value) to onCondition.
• At minimum support:
  • option = timer: Timer() > value
  • option = loudness: Loudness() > value
• If loudness input is not fully implemented yet, document phased behavior clearly (e.g., timer first, loudness as TODO or non-triggering placeholder).

4. Scheduling & Performance

• Evaluate conditions in the existing logic/event loop (no busy-wait).
• Keep per-frame checks lightweight and allocation-free where possible.
• Dispatch callbacks consistently with existing event execution model (async coroutine-based style).

5. Lifecycle & Isolation

• Condition handlers should follow existing ownership semantics (sprite/game, clones, destroy, reload).
• Behavior under stop all and related control-flow operations should be consistent with other hat events.

Acceptance Criteria

• A minimal onCondition example can be written and triggers correctly.
• event_whengreaterthan(timer, x) fires once when timer crosses threshold.
• No repeated firing while condition stays true.
• Re-fires after falling below threshold and crossing again.
• Coexists with existing events (onStart, onTimer, onMsg, etc.) without regressions.
• Add tests for:
  • Edge-trigger behavior
  • Retrigger after reset
  • Multi-instance/clone isolation
  • Stop/reload lifecycle behavior