Add parent-first-child top and parent-last-child bottom margin collapsing
using a two-level approach (Level A shifts child inside parent, Level B
adjusts grandparent for effective collapsed margin). Fix pre-existing bug
in shift_subtree_x that incorrectly skipped absolute/fixed children.
Extract shared MARGIN_EPSILON constant, add 9 unit tests and 4 golden
tests (209-212), promote 31 WPT tests to pass. Includes code review
fixes: pub(crate) visibility for is_root_element, known-limitation docs,
and inline-content shift test coverage.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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