Slate v2 Phase 9 Placeholder Matrix

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

Goal

Widen the placeholder family in the replacement matrix with the legacy custom-placeholder surface.

Scope

• add one legacy custom-placeholder row
• sync the placeholder family docs if it lands cleanly

Constraints

• do not turn this into a broader placeholder redesign
• keep the row family-level and evidence-driven
• verify through the cross-repo local runner on current port 3010

Progress

• added one legacy custom-placeholder row to the replacement compatibility matrix
• proved the row through the cross-repo local runner
• synced the scoreboard and placeholder-family docs so the landed Phase 9 slice is visible in the roadmap stack
• verification:
  • bash ./scripts/run-cross-repo-local.sh 3010 /examples/placeholder ../slate 3210 /examples/custom-placeholder "yarn build:slate-browser:playwright && yarn exec playwright test playwright/integration/examples/replacement-compatibility.test.ts --project=chromium --workers=1"