Slate v2 Roadmap Phase Read Cleanup

Supporting plan. For current queue and roadmap truth, see master-roadmap.md.

Goal

• Remove one last stale phase/read cluster from the Slate v2 docs.

Finding

• overview.md still implied the active program phase was Phase 8.
• phase7-migration-story.md still described the target audience as adopting an experimental v2 surface.
• final-synthesis.md and package-end-state-roadmap.md still described the family ledger with old not-yet-covered wording even though the current useful split is comparison-only vs intentionally later.

Patch

• Reframe Phase 8 as landed historical work.
• Reframe the migration target as a proved replacement-candidate surface.
• Align the family-ledger wording with the current public read.

Verification

• pnpm exec prettier --check on the edited plate-2 docs
• grep confirms the stale active program phase is now Phase 8 wording is gone