test(snapshot): add contention stress regressions

[riatzukiza]

Issue for this PR

Closes #74

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This phase adds high-contention snapshot regression coverage. It introduces stress tests for concurrent track/patch/diff/cleanup churn and verifies cleanup remains non-blocking while another snapshot operation holds the lock.

How did you verify your code works?

• Ran bun test test/snapshot/snapshot.test.ts from packages/opencode.

Screenshots / recordings

N/A (non-UI change).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/snapshot-phase4-stress

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