[M4] Validation Extensions for playwright_test Segments

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Milestone 4 — Playwright Test Video Integration | Priority: Medium

Extend Validator to check playwright_test segments.

• A/V drift checks for retimed videos
• Event count vs anchor count validation
• Speed factor range warnings
• OCR validation on Playwright test video
• Test failure detection in trace data
• Add to --pre-push checks

See issues/playwright-test-integration/09-validation-extensions.md for full spec.