Remove flakiness and re-enable test 11 (tests/11_collab_test.js)

Test 11 (`tests/11_collab_test.js`) is currently skipped due to flakiness impacting collaborative test operations:
- The test is currently disabled with `@disabled: true`
- This test covers collaboration workflows, window handling, and link/element operations.

**Task:**
- Diagnose and resolve the instability causing test flakiness.
- Re-enable test 11 by removing the `@disabled` flag in the file.
- Confirm robust operation for CI and local environments.

Relevant code snippets:
[tests/11_collab_test.js](https://github.com/AToMPM/atompm/blob/e988b5abcf962960c7bc5066aea9cfa2fabe7c1d/tests/11_collab_test.js)
```javascript
// TODO: temporarily disable this test for CI and local runs; re-enable after fixing flakiness
module.exports = {
'@disabled': true,
...
}
```

Typical operations involved:
- Multi-window handling (tab_a, tab_b, tab_c)
- Creation and linking of elements
- Syntax switching and client disconnects

**Check for timing issues, async handling, and environmental dependencies that may impact reliability.**

---
Labels: Test/CI/CD, bug
Issue type: Bug

Start with tests/11_collab_test.js and run test 11 in both local and CI-like environments to reproduce the flakiness. Trace the multi-window, asynchronous collaboration, linking, syntax-switching, and disconnect operations for timing or environment-dependent failures. Done means the @disabled flag is removed and the test passes reliably in local and CI runs.

Written by the indexing model from the issue text.