Close the post-await re-entrancy detection gap with AsyncLocalStorage

[camcima]

From the improvement opportunities of the 2026-08 architecture review.

Problem

Statemachine.guardSync() sets a flag around the synchronous portion of every callback, so triggerEvent / checkTransitions / whenIdle called from inside an observer, condition, or selector throw ReentrancyError instead of deadlocking. (whenIdle was added to that set in #52.)

The flag is cleared as soon as the callback yields, which is deliberate — it must not leak into concurrent external callers — but it means a re-entrant call made after a prior await inside the callback is undetectable:

sm.attachAfter({
  async notify() {
    await somethingAsync();      // guard window closes here
    await sm.triggerEvent("x");  // undetected -> permanent silent deadlock
  },
});

This is documented in src/Statemachine.ts and docs/errors.md as a known gap, and it is the more likely shape in real code: an observer that does I/O and then wants to fire a follow-up event. The failure mode is the worst available one — the machine wedges permanently and silently, and every later operation queues forever behind it.

Proposed direction

package.json requires Node >= 20, where AsyncLocalStorage is stable. Running each operation inside an ALS context and checking membership would catch re-entrancy regardless of how many awaits have elapsed.

The constraint is that the core is currently runtime-agnostic (no Node built-ins, zero dependencies), and that is worth preserving for browser/edge bundles. So make it injectable rather than imported: a guard strategy on StatemachineOptions with a synchronous default and an opt-in ALS implementation exported from a subpath (e.g. @camcima/finita/node), or auto-detected behind a feature check that bundlers can drop.

Acceptance criteria

• A re-entrant call made after an await inside a callback throws ReentrancyError rather than deadlocking, when the strategy is enabled.
• Default build pulls in no Node built-ins and keeps the zero-dependency guarantee.
• Concurrent external callers are still never flagged (the existing test for this must keep passing).
• docs/errors.md gap note updated to describe when the gap is and isn't closed.