Add closest, classList, and dataset to the DOM polyfill

[airhorns]

What changed

Supersedes #619, carrying @developit's six commits unchanged (authorship intact) and rebasing them onto current main (post-#620/#625/#653). His PR description covers the core design: closest(), classList, and dataset composed from the polyfill's existing selector, class-name, and attribute primitives, with no new or widened hooks.

On top of that, four commits fix issues found while running this branch under a production embedder (each with the failure it prevents):

1. dataset keys now enumerate — the proxy gained has/ownKeys/getOwnPropertyDescriptor traps, so Object.keys(el.dataset), spread, in, and for...in see the data attributes instead of always coming back empty.
2. Inherited Object.prototype members stay visible — the traps only claim actual data-* names, so `${el.dataset}`, .toString, and hasOwnProperty work instead of throwing on string coercion.
3. Names that don't round-trip camel-casing are hidden — data-fooBar is excluded from ownKeys per the HTML spec's dataset name rules, which also prevents data-fooBar + data-foo-bar from producing duplicate keys and a permanent proxy-invariant TypeError.
4. matches()/closest() route through the recursive selector matcher — descendant combinators (article span) now check all ancestors instead of only the immediate parent, and the dead flat matcher is deleted.
5. Object.defineProperty(el.dataset, …) routes through the named setter — a data descriptor writes the attribute (WebIDL [[DefineOwnProperty]] semantics) and accessor descriptors are rejected, instead of the default trap defining a non-configurable property on the proxy target and poisoning every later enumeration.
6. Object.preventExtensions/freeze/seal throw like a real DOMStringMap — the proxy refuses [[PreventExtensions]] as WebIDL requires of legacy platform objects, instead of letting the target become non-extensible and turning every later enumeration into a proxy-invariant TypeError.

Tests

The original coverage plus 8 new tests: dataset enumeration as own properties, inherited-member visibility, defineProperty routing and accessor rejection, preventExtensions refusal, non-round-tripping name hiding, and matches/closest descendant-combinator traversal.

Validated with:

• pnpm exec vitest run (263 tests)
• pnpm type-check
• pnpm lint
• pnpm --filter @remote-dom/polyfill build

🤖 Generated with Claude Code