docs: Phase 1 plan (per-cell table header) + spec phase-boundary fix

Correct the spec's over-claim that a Phase-1 build round-trips colspan/rowspan
through the composer (it can't until Phase 2's sparse-grid work); received
messages already render via V2 independent of the editor. Add the full TDD
Phase 1 implementation plan for per-cell header/highlight.

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