fix(js): expose DOMStringMap globally

[lisa0314]

What changed

• expose DOMStringMap as a non-enumerable global WebIDL interface
• use an internally constructed DOMStringMap instance as the dataset Proxy target
• preserve live data-* property access while allowing prototype properties and symbols to resolve normally
• keep direct DOMStringMap construction illegal while restoring constructor identity, instanceof, and [object DOMStringMap]

Previously, Element.dataset used a plain object as its Proxy target and the runtime did not define window.DOMStringMap. Dataset reads and writes worked, but constructor feature detection and prototype identity did not.

Validation

• red baseline reproduced an undefined interface, false prototype checks, and [object Object]
• cargo nextest run -p obscura-js: 281 passed
• cargo build --release
• issue reproduction now returns function for both global checks and true for instanceofResult
• git diff --check

Rendering

Not applicable. The change is limited to the JavaScript DOM interface and does not affect layout, paint, screenshots, or rendering fixtures.

Performance

No standalone performance benchmark was run. The existing per-element dataset Proxy cache remains in place, and the change adds no work to layout, paint, network, or page task loops.

Checklist

• The change is focused and preserves existing live data-* behavior.
• Regression coverage includes the global constructor, branding, prototype identity, symbols, and reads and writes.
• The complete obscura-js suite and release build passed.
• Rendering behavior is unaffected.
• Performance implications were reviewed without identifying a hot-path expansion.

Fixes #599.