fix: preserve inheritable defaults in referenced subtrees

[Neallin-917]

Fixes #2239.

When an element with an id is instantiated through <use>, an inheritable default inside its subtree can prevent the instance from taking an unintended inherited value from the <use> context. Preserve only inheritable defaults in those subtrees; ordinary default attributes remain removable.

Adds a focused regression fixture for fill-opacity="1" beneath an id target used from a fill-opacity=".3" context.

Validation:

• pnpm lint
• pnpm typecheck
• pnpm test (518 passed, 3 skipped)
• pnpm test:bundles
• local regression suite completed optimization and comparison with 4,905 expected matches; the local renderer reported 32 shouldHaveMismatched differences and no shouldHaveMatched differences in unrelated Oxygen/W3C fixtures.

Disclosure: AI assistance was used to investigate and implement this contribution.