Tree-walking ECMAScript interpreter built on ecma-syntax-cat. v0.5.0 adds the microtask driver (async track chunk 2 of 5-7): `promise::resolve(id, value, heap, fuel)` / `reject(...)` transition a Pending promise and recursively drain its queued `.then` / `.catch` handlers, settling each chained child via its callback (callback throws become Rejected; missing callbacks pass the value through). Test hooks `__resolve_promise(p, v)` / `__reject_promise(p, v)` are installed via `promise::install_test_hooks(env, heap)` so the driver is exercisable from JS ahead of the `Promise.resolve` / `await` surface in chunks 3-4.