Slate v2 async decoration caret cluster proof

Goal

Process Cluster 10 from the Slate issue ledger:

• issue: #5987 caret jumps when decorate changes from an async state update
• upstream PR: #6033 fixes the same failure by keeping decoration restructuring and Editable selection restoration in sync

Evidence

• The issue recording shows a caret at the end of This is some text here about. there, then typing there, then a delayed highlight appears and the caret jumps backward between about. and there.
• Existing ledger state only claims Improves #5987 because current proof covers projection-source stability but not the exact async Editable.decorate repro.
• The relevant Slate v2 surface still supports Editable.decorate; exact proof should target that adapter, not only first-class decorationSources.

Claim Bar

Promote to Fixes #5987 only if a browser integration test proves all of:

1. Editable receives a decorate prop whose function identity changes after an async state update.
2. The user types matching text at the end of the editor before the async decoration applies.
3. The delayed decoration visibly restructures the rendered text.
4. Browser selection and Slate selection remain at the typed document end after the delayed decoration applies.

If any part is missing, keep the issue as Improves #5987.

Plan

1. [done] Add a focused decorations-async example that mirrors the upstream issue path.
2. [done] Add a Playwright regression around typing at the end, waiting for delayed highlight, and asserting caret stability.
3. [done] Fix slate-react only if the new row fails.
4. [done] Run focused browser verification for the new row.
5. [done] Update Cluster 10 ledgers with the exact proof and final claim.

Changeset

No changeset is required if this pass only adds a site example, tests, and ledger updates. If package runtime code changes under .tmp/slate-v2/packages, decide changeset necessity before closeout.

Package runtime code changed under .tmp/slate-v2/packages/slate-react, so the Slate v2 checkout includes .changeset/async-decorate-caret.md. No Plate changeset is required.

Result

Claim: Fixes #5987.

The new browser row reproduced the exact failure before the runtime fix:

• Slate model selection stayed at offset 41.
• Browser DOM caret stayed at offset 35, where the old decorated text ended.

The fix asks the editable repair runtime to force a repair render after external Editable.decorate refreshes. Decoration-only DOM changes do not create an editor commit, so this gives selection export a render pass after the projected text DOM has been restructured.

Verification

• bun lint:fix
• bun --filter slate-react typecheck
• PLAYWRIGHT_RETRIES=0 bun playwright playwright/integration/examples/decorations-async.test.ts --project=chromium