Keep table rails pointer-transparent and clipped to the document

The rail container now passes the pointer through, so the paragraph above
a table stays clickable; only grips, shown buttons and the seams beside the
pointed track take it. Rails clip to the document scroller, the remove
button hides while an insert is aimed, pointed-cell updates skip no-ops,
and the grip glyph uses spacing tokens.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>