docs: design spec for richtext table header/highlight + colspan/rowspan

Per-cell header flag and cell merging (colspan/rowspan) for the RichTextEditor
composer, with the InstantPage V2 viewer verified. Two-phase plan: per-cell
header first (low risk, mirrors the alignment seam), then span/merging.

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