In-process async vfs runs

Drain a controlled microtask/timer loop inside runNodeInProcess so async pure-JS node -e/node <file> can run on the no-spawn vfs path instead of bailing to native on any Promise result.

Why not: Marginal value, real correctness risk, doesn't extend naturally. Today runNodeInProcess bails to native the moment a run returns a Promise, so the in-process path only catches trivial sync node -e. Spinning a controlled event loop would only save node-boot (~50–100ms) over the already-shipped os fork — and it's gated hard by the differential-correctness suite, which a half-real event loop is liable to break: unhandled rejections, timer leaks, and require-cache bleed across runs all diverge from a real node process in ways the gate must catch. The os backend already runs async tools faithfully and isolated.

Revisit only if a profile shows pure-JS async invocations are a frequent, hot slice of a real loop and a spike proves a drained-loop runtime holds the differential gate (rejections, timers, require-cache isolation) — spike-first, gate-green before any production wiring.