Add indexed and named NamedNodeMap access

[andrewiggins]

What changed

Add live browser-style property access to the polyfilled NamedNodeMap without making it inherit from Array:

element.attributes[0];
element.attributes.id;

• Resolve indexed and named reads through one shared proxy in the prototype chain, preserving NamedNodeMap identity and avoiding a proxy allocation for every element.
• Match getNamedItem() by qualified name across namespaces while preserving methods and inherited properties.
• Parse only canonical ECMAScript array-index property names, rejecting fractions, aliases, and values outside the 32-bit array-index range before slower string coercion or linked-list traversal.
• Keep Attr.ownerElement, linked-list state, and ownerDocument correct when attributes are removed, replaced, directly adopted, or carried across documents with an adopted or inserted subtree. Directly adopted attributes are detached from their current element before their document changes.
• Reject attempts to attach an attribute already owned by another element.
• Add a minor @remote-dom/polyfill changeset.

Design and performance

A constructor-returned proxy prototype was benchmarked first, but it substantially deoptimized existing methods and added about 32 bytes of retained V8 heap per materialized map. The shared prototype fallback restores ordinary getAttribute, mutation, length, item, iteration, and serialization paths to approximately the pre-proxy baseline while retaining indexed and named reads.

The strict index parser was benchmarked against simpler coercion and bitwise alternatives. Number.isInteger() plus explicit bounds was the clearest option and performed at least as well overall, while avoiding duplicate linked-list traversal for unusual property names.

The final size-limit run reports @remote-dom/polyfill at 7.05 kB Brotli (+218 B, 3.19%) against an 8 kB limit.

Web Platform Tests

• Enable six supported cases in dom/nodes/attributes-namednodemap.html; defer two cases that still require Document.createAttribute(), with one also requiring an exposed NamedNodeMap constructor.
• Promote the Document-getElementById.html case that mutates an ID through element.attributes[0].value.

Current classified WPT result: 5/5 files passed with no supported failures or inventory drift.

Intentional scope

The shared prototype fallback provides ordinary indexed and named reads, but it is not a complete Web IDL legacy-platform-object implementation. This change does not add has, own-key/descriptor reflection, assignment/deletion restrictions, alternate Reflect.get() receiver behavior, or indexed precedence over properties placed directly on NamedNodeMap.prototype. In-use attributes currently throw a basic Error because the polyfill does not expose DOMException.

Document.createAttribute(), global NamedNodeMap exposure, complete namespace/local-name modeling, and HTML case-folding remain follow-up work.

Validation

• pnpm type-check
• pnpm exec vitest run --project polyfill --project wpt-runner — 16 files, 136 tests passed
• pnpm test:wpt — 5/5 classified files passed
• pnpm lint
• pnpm size