feat: wait for external events via .waitForEvent + engine.sendEvent

[danfry1]

Summary

Adds event-driven durable suspension — the second half of durable suspend, alongside .sleep(). A workflow can pause until an external signal arrives (a webhook, an approval, a payment confirmation, a human action):

.step('create-payment-intent', async ({ input }) => ({ intentId: await stripe.createIntent(input.orderId) }))
.waitForEvent('payment.succeeded', { schema: z.object({ chargeId: z.string() }), timeoutMs: 30 * 60 * 1000 })
.step('fulfil', async ({ prev, input }) => ship(input.orderId, prev.chargeId)) // prev = the event payload

// ...later, from a webhook handler:
await engine.sendEvent(runId, 'payment.succeeded', { chargeId: event.data.object.id })

A run reaching the wait is persisted as waiting with its lease released, and resumes — on any engine instance — when the event is delivered. The (optionally schema-validated) payload becomes the next step's prev.

Design

• New waitForEvent execution unit (mirrors .sleep), so it fits the replay model with no determinism constraints.
• Durable, order-independent delivery. Events are persisted (a workflow_events table / in-memory buffer). An event sent before the run reaches the wait is buffered and consumed when it gets there.
• Race-free. A deliver-during-suspend race is closed by having waitRun check, in its own transaction, for an already-buffered event and stay reclaimable if one exists.
• Timeouts reuse the wake_at machinery; on expiry the run fails with the new WaitTimeoutError.
• engine.sendEvent validates the payload against the wait's schema before delivering.

Surface

• RunStatus / StepStatus gain waiting. New exported WaitTimeoutError.
• StorageAdapter gains waitRun / deliverEvent / takeEvent; claimNextRun now also wakes due waiting runs. Implemented across all four adapters (SQLite adds a workflow_events table, created via CREATE TABLE IF NOT EXISTS — no migration needed).

Tests

• wait-for-event.test.ts (10): suspend/resume, early-arrival buffering, timeout → WaitTimeoutError, in-time delivery, schema validation, unknown-event / missing-run, cross-engine crash recovery, sequential waits, cancel-while-waiting, duplicate-name.
• Adapter contract tests for waitRun / deliverEvent / takeEvent + claim-wake across memory / better-sqlite3 / node:sqlite, plus the buffered-event race case. Bun smoke test extended for the new methods.

404 tests pass; typecheck, build, coverage (85.4% branches), lint:deps, and test:bun all green.

Base

Stacked on #51 (durable sleep) — it reuses the suspend/wake_at/claim-wake machinery. Once #51 (and its base #52) merge to main, this retargets to main. StepStatus/RunStatus already union sleeping + waiting here.