feat: add slate-yjs collaboration package

[felixfeng33]

Summary

Adds a first-party slate-yjs collaboration package on top of the Slate v2 extension/runtime stack. The package binds Slate editors to a shared Yjs document root, exports local commits into Yjs, imports remote Yjs events through editor updates, and keeps cursor/selection traffic in awareness.

This PR is opened from the current yjs checkout as-is, so it includes the existing v2 branch stack plus the new slate-yjs package work. The slate-yjs review entry points are packages/slate-yjs, site/examples/ts/yjs-collaboration.tsx, and playwright/integration/examples/yjs-collaboration.test.ts.

Fixes ianstormtaylor#3715

What changed

• Adds packages/slate-yjs with core, react, and internal exports.
• Adds codec/controller/awareness tests plus React hook coverage.
• Adds a full two-editor Yjs collaboration example with deterministic controls.
• Wires source-first TypeScript paths, workspace dependency, docs, resources, and changeset metadata.
• Adds Chromium coverage for editor convergence, undo/redo, pause/resume, and cursor projection.

Verification

• bun check
• bunx playwright test playwright/integration/examples/yjs-collaboration.test.ts --project=chromium
• dev-browser --connect http://127.0.0.1:9222 against /examples/yjs-collaboration

[changeset-bot]

🦋 Changeset detected

Latest commit: b0504f6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
slate-dom	Major
slate-react	Major
slate	Major
slate-yjs	Minor

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