fix: improve parity capture diagnostics

[jan-kubica]

Summary

• hide editor and playground chrome that overlaps element screenshots
• capture a bounded tail of playground startup output
• fail immediately when the playground process exits before readiness
• include startup output in parity infrastructure errors

Verification

• 24 focused parity extraction tests passed
• visually verified clean page-only screenshot output on an anonymized fixture
• bun audit reported no vulnerabilities
• lint and typecheck intentionally delegated to CI

CC on behalf of @jan-kubica

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 1e3ee0a9-a4f5-4b3c-9654-da926ddec285

📥 Commits

Reviewing files that changed from the base of the PR and between 4aa42ec and c731293.

📒 Files selected for processing (2)

• parity/__tests__/folioExtract.test.ts
• parity/folioExtract.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/parity-screenshot-diagnostics

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[gemini-code-assist]

Code Review

This pull request refactors screenshot styling into an exported constant and improves playground startup diagnostics by capturing and reporting stdout/stderr logs on failure. The review feedback highlights a critical deadlock and background promise leak during server startup, as well as a missing null/undefined guard for the output stream in captureOutputTail.

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