Fix reactive core effect lifecycle: track signals, restore tracking context

[nakasyou]

Summary

Fixes the standalone effect/signal in core/reactive/core.ts (exported via eclipsa/client), which had a broken lifecycle:

• signal.get() added the effect to signal.effects but never added the signal to effect.signals, so the FinalizationRegistry cleanup that iterates effect.signals was a permanent no-op → effects were never unsubscribed.
• effect() set currentEffect without try/finally: a throwing body left tracking dangling (next unrelated get() subscribed the dead effect), and successful runs reset to null instead of the previous effect, breaking nested effect() inside an outer effect.
• The registry target was fn, but the held value (newEffect) references fn, so fn could never be collected and cleanup could never fire.

Changes:

get() { if (currentEffect) { this.effects.add(currentEffect); currentEffect.signals.add(this) } }

effect(fn): () => void {
  const previous = currentEffect
  currentEffect = newEffect
  try { newEffect.fn() } finally { currentEffect = previous }
  const dispose = () => { registry.unregister(dispose); unsubscribeEffect(newEffect) }
  registry.register(dispose, newEffect, dispose)   // token is the dispose handle, not fn
  return dispose
}

effect now returns a dispose function for explicit unsubscription; GC-based cleanup runs when the caller drops the handle.

Adds unit tests for dispose/unsubscribe, throwing bodies, and nested effects.

Link to Devin session: https://app.devin.ai/sessions/50e38e97b4534695ba2c96c67ba6c585
Open in Devin Desktop: https://app.devin.ai/desktop/session/50e38e97b4534695ba2c96c67ba6c585?variant=devin
Requested by: @nakasyou

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Website preview: https://pr-131-eclipsa.xiarenda61.workers.dev

Commit: 8757e5b

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