Fase 16.Z. Documents the component decisions, the five integration
sites, the unit + e2e test surface (5 + 3 specs), the vitest browser-
condition tweak, the Supabase storage-key derivation gotcha used by
SP-E-03, and the cumulative MVP2 test count (497 → 505).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
