feat(core): group undo steps by browser task

[ilyamore88]

What

This PR implements undo-group-by-task, proposed in #201. It is the middle of the stack: #201 (proposal) ← this ← #191 (clipboard paste proposal).

UndoRedoManager (core)

• Task buffering. Local model events are buffered for the current browser task. The task end is detected with setTimeout(0), which runs after the task and its microtasks.
  • One event in the task (a keystroke) uses the existing consecutive-text batching, so typing is unchanged.
  • Several events in the task (Enter split, paste) close the open batch and start a new step together. Typing after such a step may still continue it under the text rule, so "replace selection and keep typing" is one step. Typing before it never merges in.
• Buffer handling. undo() and redo() record buffered events first. destroy() clears the task timer and drops the buffer.
• Fix: block undo/redo. Block-level events carry a single serialized block, but EditorDocument.insertData/removeData expect a list for a BlockIndex. Undoing or redoing any block add or remove threw data.forEach is not a function. #apply now wraps the block in a one-item list.

Tests

• Unit tests for every spec scenario.
• The existing "should NOT batch …" tests now call endTask() between events that model separate user actions.
• UndoRedoManager.integration.spec.ts (real model and BlocksManager): a mid-paragraph Enter undoes in one step, and a split that adds a block undoes and redoes without AlreadyExistingKeyError.

Docs

• docs/collaboration.md and docs/diagrams/undo-redo-flow.mmd describe task grouping, and note that collaboration-manager doesn't group by task yet.

tasks.md is fully checked in this PR.

Verification

• packages/core: 197 tests pass, lint:ci is clean, and the build succeeds.
• Stryker on UndoRedoManager.ts scores 86%.
  • The one survivor in the new code is equivalent: scheduling the task timer on every event behaves the same as scheduling it once.
  • The other survivors are in code this PR doesn't change.
• Root yarn lint passes. Root yarn test passes except collaboration-manager/CollaborationManager.spec.ts, which fails to load locally on Node 20.9 (WebSocket is not defined). That package doesn't depend on core.

Notes for reviewers

• Archive gate. The OpenSpec archive gate will fail here until the change is archived, because tasks.md is complete. Archive it before merging, or label the PR openspec:defer-archive while the stack is under review.
• Behaviour change. Any action that makes several model changes in one task now undoes in one step.

[github-actions]

Unit Tests

Package	Coverage	Delta
@editorjs/core	73.87%	+0.73% 🟢
@editorjs/editorjs	100%	0% ⚪️

Mutation Tests

Package	Mutation score	Dashboard URL
@editorjs/core	86.14% 🟢	Dashboard