[M4] Pipeline Integration for Playwright Tests

[cursor]

Milestone 4 — Playwright Test Video Integration | Priority: Medium | COMPLETED in PR #27

Added playwright_test stages to Pipeline.run(): test execution → trace extraction → sync → compose.

• --skip-playwright-tests flag for generate-all
• Only runs stages when playwright_test segments exist in visual_map

Implemented in src/docgen/pipeline.py. See issues/playwright-test-integration/06-pipeline-integration.md.

[jmjava]

Library note (merged): manifest_from_mapping for non–Playwright-test consumers

PR: #52

Downstream pipelines that integrate docgen but do not have Playwright test specs (or pytest markers) can build demo manifests in-process via docgen.manifest_from_mapping → render(), instead of relying only on filesystem manifests or TS/pytest discovery.

Complements existing pipeline playwright_test stages: mixed repos can document CLI segments programmatically while browser segments use the test-runner path when present.