Detach adopted attributes from their elements

[andrewiggins]

Summary

Detach an attached Attr when passed to Document.adoptNode(), including same-document adoption.

• Remove the exact attribute by identity through a shared internal NamedNodeMap removal helper.
• Commit detachment and destination ownership before removal hooks or custom-element callbacks run.
• Preserve reentrant reattachment and third-document adoption; notification errors leave ownership coherent.
• Keep ordinary map removal behavior and generic subtree adoption unchanged.
• Add focused regression tests and a patch changeset for @remote-dom/polyfill.

Stack and scope

Stacked on #682 (adopt-inserted-subtrees), which owns generic subtree adoption. This is the focused attached-Attr follow-up split from #693 and does not depend on indexed/named NamedNodeMap property access. Tests obtain attributes through getNamedItem() / getNamedItemNS().

Validation

• Focused attached-Attr tests: 12 passed.
• Existing subtree adoption tests: 13 passed.
• Existing NamedNodeMap and custom-element reaction suites: 27 passed.
• Full unit suite: 558 tests passed across 48 files.
• pnpm type-check, pnpm lint, pnpm build, pnpm size, and git diff --check: passed.
• Polyfill bundle: 11.22 kB against the existing 12 kB limit.
• Playwright: 13 tests passed on an unchanged retry; the initial run had four iframe interaction timeouts.
• Verified native Chrome detaches same-document attached attributes, exposes destination ownership to removal callbacks during cross-document adoption, and preserves a callback's third-document adoption. Also verified the built polyfill's callback ownership and reentrant transfer behavior.

No existing pinned WPT test was found that directly adopts an attached Attr, so this PR does not change WPT classifications. Related adoption WPTs remain blocked by unrelated APIs/behavior. Polyfill-specific hooks and exception propagation are covered by unit tests, not claimed as native-browser equivalents.