test(codemode): run test262 files verbatim from a local checkout

[rekram1-node]

Adds a runner that executes upstream test262 files verbatim against the interpreter, so language and built-in behavior is checked against the spec's own tests instead of hand-written paraphrases. The test files themselves are not committed: script/sync-test262.ts copies them from a local checkout at a pinned revision into gitignored directories. Without them the runner registers zero tests, so CI is unchanged.

What's here

• test/test262/manifest.json — pinned revision (250f204f), directories to copy (built-ins/Array/prototype, language/statements), and what is left out: unsupported flags/features/includes, plus boundaries — code patterns for intentional interpreter limits (classes, this, arguments, call/apply/bind, prototype objects, descriptors, accessors, boxed primitives, sloppy mode, eval, Symbol(), new.target, $262).
• script/sync-test262.ts — copies from a checkout, verifying it is at the pinned revision and applying the manifest. 3993 files, 5.6 MB, all gitignored.
• test/test262/run.ts — runs one file: prepends "use strict" (sloppy-only tests are never copied), provides the harness (assert, Test262Error, compareArray, $DONE, $DONOTEVALUATE) as host globals, drains the job queue for async tests, and interprets negative.
• test/test262.test.ts — one bun test per file present. Files in skipped.txt fail on a known gap and are skipped.
• script/test262-report.ts — runs everything including skipped files, groups failures by cause, reports skipped files that pass again, and --write regenerates skipped.txt.
• interpreter-support.md — one [ ] item per gap category found (18).
• src/interpreter/{runtime,execute}.ts — optional extraGlobals hook so the harness can be host-provided (assert.sameValue is a property on a function, which programs cannot express).

Results at the pinned revision

3252 pass, 741 skipped, 0 fail; ~3 s for the full set.

Largest gaps (all in skipped.txt with reasons): function name/length (238), const { constructor } = err reading undefined (154), async generator parameter evaluation timing (95), interpreter-raised errors typed Error instead of TypeError/RangeError/ReferenceError (94), argument coercion in array methods (42), arr.length = n (21).

Follow-ups: extend the manifest to the remaining directories we implement (Object, JSON, String, Number, Math, Date, Map, Set, Promise, RegExp, language/expressions); decide later whether and how to vendor the files for CI.