Add closest, classList, and dataset to the DOM polyfill

[developit]

What changed

Remote code can now use three familiar DOM conveniences:

• closest() starts with the element itself and walks parentElement, using the selector matcher already shared by querySelector().
• classList provides live token operations and read-only indexed access over className.
• dataset maps camel-cased properties to data-* attribute reads, writes, and deletes.

dataset lives on the shared Element abstraction. Remote DOM deliberately keeps its element model flat, so this makes the API available to ordinary and custom elements without changing document.createElement(), constructor identity, or the existing prototype hierarchy. Each element lazily creates and caches one dataset proxy, preserving normal dataset object identity while keeping it live against attribute changes.

classList is also cached per element. DOMTokenList.prototype inherits from a shared proxy that supplies live numeric properties such as classList[1]; numeric assignments are silently ignored. Internal state uses the polyfill's existing owner and value symbols. This avoids allocating a proxy per element and does not pretend the token list is an array or expose mutable array methods.

These APIs do not add or widen any hooks. Class and data changes continue through the existing attribute methods, so remote synchronization sees exactly the same mutations it did before.

Tests

Added focused coverage for closest-ancestor traversal, live and indexed class updates, token operations, stable class-list and dataset identity, dataset name conversion, string coercion, deletion, and attribute hook calls.

Validated with:

• pnpm exec vitest run (184 tests)
• pnpm type-check
• pnpm lint
• pnpm --filter @remote-dom/polyfill build on Node 20.20.0