feat(codemode): add the Headers built-in

[rekram1-node]

First of the fetch types (Headers, then Response, then Request), each its own PR. A program object wrapping the host's Headers, the same way URL and URLSearchParams wrap theirs: Bun validates, normalizes, folds case, combines values, and orders iteration; the runtime only converts at the edges.

const h = new Headers({ "Content-Type": "application/json" })
h.append("Accept", "text/html")
for (const [name, value] of h) ...
Object.fromEntries(h)           // { accept: "text/html", "content-type": "application/json" }

program                      host
HeadersObj ───── wraps ────▶ Headers
  new Headers(record | pairs | Map | Headers)
  append delete get getSetCookie has set forEach keys values entries
  for...of, spread, destructuring, yield*   (live, sorted by name)
  String(h) → "[object Headers]"    JSON / result / tool arg → { name: value }
  console.log(h) → Headers(2) {"accept":"…","content-type":"…"}

boundary   Headers ⇄ HeadersObj as copies, both directions

 src/stdlib/
+  headers.ts        118 lines: constructor, methods, host TypeError → program TypeError
   url.ts            readPairs(ctx, init, label) shared with URLSearchParams (was inline)
 src/interpreter/
   objects.ts        HeadersObj, isWrapper
   intrinsics.ts     Headers prototype
   interpreter.ts    iterate / yield* over Headers
   extensions.ts     toHost / fromHost
 src/stdlib/console.ts, value.ts, data.ts, references.ts   printing, coercion, JSON, describe

Left out on purpose: Symbol.iterator (no custom Symbols), iterator objects from keys()/values()/entries() (arrays, as with URLSearchParams).

Tests

• test/stdlib.test.ts: construction, mutation, iteration everywhere, serialization, every error path.
• test/extensions.test.ts: Headers cross as copies in both directions.
• test/web-wpt.test.ts: six fetch/api/headers/*.any.js files adapted near-verbatim over a small testharness.js shim, attributed in the header. Two set-cookie cases dropped because Bun's own Headers sorts set-cookie2 ahead of set-cookie.

check	result
packages/codemode bun test	5136 pass / 0 fail (+11)
test262	3858 pass / 426 known gaps (unchanged)
bun run check	35/35

interpreter-support.md updated; Request, Response, Blob listed as open.

README

Second commit. The Values section described an export that no longer exists; it is now an Extension.make section, which the hooks paragraph already referred to without introducing. The "differs from JavaScript" bullet no longer says fetch is unavailable, since a host may add it.