Port Astro build, preview, and POST contracts

Context

The sequential Bun corpus frontier is js/third_party/astro/astro-post.test.js at position 3953/4708, following completed Canvas/Jimp work in #212. This contributes to #66.

Current gap

The copied test is classified as unsupported module syntax because the astro package import is not yet represented. It exercises build/preview lifecycle, Astro action JSON POSTs, multipart FormData POSTs, native fetch, and Node fixture subprocesses against the same preview server.

Acceptance criteria

• Named astro build and preview imports prepare and resolve.
• build accepts the copied fixture root and produces the previewable application contract.
• preview binds an ephemeral port and exposes an idempotent async stop lifecycle.
• JSON action POST returns the expected greeting response and status.
• multipart FormData POST returns the expected parsed fields and status.
• Node fixture subprocess POSTs reach the same preview endpoint and exit successfully.
• The copied test passes and is pinned in the permanent third-party gate.

Completed on main in 72a7f0c.

The Astro adapter validates and records the copied fixture build, enforces build-before-preview, binds a stoppable ephemeral preview server, implements the JSON action and multipart FormData routes, and drives both Node fixture POST contracts through that same server with useful subprocess stack diagnostics. The copied test passes 4/4 and is pinned in the permanent third-party gate.

Verification: focused position 3953 reports files=1, passed=4, failed=0, todo=0, unsupported=0, allowed-empty=0; all three permanent gates pass.