feat(richtext): table per-cell header/highlight + colspan/rowspan

Per-cell header (highlight) flag replacing the row-level header, plus
colspan/rowspan cell merging in the RichText composer — create + view
(InstantPage V2 already renders received merged/header tables). Covers
the Core span model + covering map + transforms, TableBlockBox span-aware
geometry, the cell-rectangle selection + Merge/Split UI, and the
ChatInputContent/InstantPage round-trips.

Squashed from the feature/richtext-table-header topic branch.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>