Prototype prebuilt npm binary packaging

[benvinegar]

Summary

• add phase-1 prebuilt npm packaging scripts and a host-package staging flow
• teach the hunk launcher to prefer installed prebuilt binaries before falling back to the current Bun-backed runtime
• add Linux/macOS CI coverage that stages, verifies, and smoke-installs the prebuilt packages without Bun on PATH

What this lands

This is the first shipping phase of the prebuilt-binary plan from docs/prebuilt-binaries-plan.md.

It does not switch the published npm package over yet. Instead it gives us a working, validated packaging path inside the repo:

• scripts/stage-prebuilt-npm.ts generates:
  • a tiny top-level hunkdiff package
  • the current host's platform package (for example hunkdiff-linux-x64)
• scripts/check-prebuilt-pack.ts verifies the staged tarballs
• scripts/smoke-prebuilt-install.ts proves a clean global install works without Bun on PATH
• bin/hunk.cjs now prefers an installed prebuilt binary package, but still falls back to the existing Bun-backed path so current releases keep working during rollout

Validation

• bun run typecheck
• bun test
• bun run test:tty-smoke
• bun run check:pack
• bun run build:prebuilt:npm
• bun run check:prebuilt-pack
• bun run smoke:prebuilt-install

Follow-up

A later PR still needs to wire up the full multi-platform publish workflow:

• artifact fan-in
• platform package publish order
• tag-driven npm release automation
• wider platform matrix (musl / Windows / extra variants as needed)