Slate v2 Roadmap Public README Alignment

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

Goal

• Remove one last public claim contradiction from the Slate v2 docs.

Finding

• The root README still called Slate an "experimental replacement-candidate stack".
• That was stale against the current release-readiness read:
  • credible replacement candidate
  • not yet an honest blanket replacement

Patch

• Align the root README with the current public verdict.
• Keep the narrow anchor-surface warning.
• Do not reopen the broader roadmap stack for this.

Verification

• yarn prettier --check /Users/zbeyens/git/slate-v2/Readme.md
• grep confirms the stale experimental wording is gone from the public docs