Slate v2 Roadmap Historical Wording Cleanup

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

Goal

• Remove two leftover historical phrases that still sounded more live than they should.

Finding

• core-foundation-spec.md still called the transaction seam "experimental" even though the public transaction seam is already a real current surface.
• slate-batch-engine.md still said "the right next slice" without marking it as historical.

Patch

• Reframe the core-foundation seam as provisional instead of experimental.
• Reframe the batch-engine wording as the right slice at that point.

Verification

• pnpm exec prettier --check /Users/zbeyens/git/plate-2/docs/slate-v2/core-foundation-spec.md /Users/zbeyens/git/plate-2/docs/slate-v2/references/slate-batch-engine.md /Users/zbeyens/git/plate-2/docs/plans/2026-04-07-slate-v2-roadmap-historical-wording-cleanup.md
• grep confirms the stale historical wording is gone from the target docs