Port Express response Location contracts

Context

The sequential Bun corpus frontier is js/third_party/express/res.location.test.ts at position 3966/4708, following JSON response work in #223. This contributes to #66.

Acceptance criteria

• res.location() is chainable and sets Location for strings, URL objects, and coerced values.
• Unicode, spaces, data URIs, relative URLs, and already-encoded sequences are encoded without changing authority or trailing-slash semantics.
• Location values remain compatible with legacy url.parse host validation.
• The copied suite passes with existing todos and is pinned in the permanent third-party gate.

Completed on main in 06ec64e.

res.location() is now chainable and handles strings, URL objects, and coerced values through URI-safe header encoding. It preserves explicit trailing-slash intent and existing percent octets while encoding Unicode, spaces, data URIs, and relative paths without changing authority semantics used by legacy url.parse.

Verification: focused position 3966 reports files=1, passed=12, todo=10, failed=0, unsupported=0, allowed-empty=0; all permanent copied-corpus, SQL, and third-party gates pass.