fix(core): refuse effect/listener/plugin registration while a fiber is UNLOADING - #96

Summary

Fiber.assertActive() only checked uid !== null — "not disposed", not
"not unloading". During a deactivation (a requirement is withdrawn and the
fiber survives as PENDING), an undo that calls ctx.effect(...) was
accepted, and the resulting disposer leaked permanently:

1. the new effect executed and its disposer was pushed into
   fiber._disposables — after _unload already clear()ed the list;
2. the fiber ended PENDING while still holding an effect
   (fiber.getEffects().length > 0 on an inactive fiber);
3. even a later fiber.dispose() never ran that disposer, because the epoch
   is already INACTIVE and no further unload is triggered — permanent
   residue.

Fix

The UNLOADING lifecycle check is added to a new assertRegistrable() used
by the registration entry points — effect(), on(), plugin() — and
NOT to assertActive() itself, because update() and restart() must keep
working through an inertial reload's UNLOADING pass (a broad guard broke
the existing "update config while injected service reloads" test).

assertRegistrable() {
  if (this.uid !== null && this.state !== FiberState.UNLOADING) return
  throw new CordisError('INACTIVE_EFFECT')
}

Repro / regression test

packages/core/tests/fiber.spec.ts — "refuses effect registration while
UNLOADING (G5 residue)": a provider dispose withdraws svc, the consumer's
generator-effect undo calls ctx.effect() during teardown. Before the fix
leaked === true and the fiber holds 1 leaked effect; after, leaked === false, getEffects().length === 0, state PENDING.

Full core suite: 71 passed.

Notes

• This is the revl-requested hardening (docs/backends-roadmap.md item 74(a)):
  revl's TS tier pins this exact behavior. Related PR feat(core): make effect disposal and Fiber lifecycle reentrant-safe #39 (reentrant fiber
  lifecycle) guards effect() directly; this change places the guard on the
  shared registration path so on()/plugin() inherit it. The two can be
  reconciled during feat(core): make effect disposal and Fiber lifecycle reentrant-safe #39's review.
• FAILED fibers still pass (as before): update() on a failed fiber is a
  legitimate recovery path.