feat(demo-function): address GitHub issue #47 (Playwright test mode, VHS, caching)

[jmjava]

Summary

Completes the outstanding pieces of issue #47 for docgen demo-function:

• CLI: --manifest and --output (directory); --output-dir remains as a hidden backward-compatible alias.
• Playwright test recording (F2): YAML can set demonstration.spec + demonstration.grep; docgen runs npx playwright test with --video=on and viewport from output_budget.resolution (F3).
• CLI / VHS: kind: cli requires demonstration.tape; renders via existing VHS runner (VHSRunner.render_tape_at), resolves tape/output relative to the manifest directory using Config.minimal.
• Caching: cache key uses sha256(fn_source_sha + intent_sha + fixture_sha) per the contract (first 16 hex chars stored).
• Assertions: assertions_to_surface are burned in as bottom subtitles (ffmpeg WebVTT) on the visual before optional TTS mux.
• Length cap: final rendered.mp4 is trimmed to output_budget.duration_seconds (hard cap 60s unchanged).
• F1 / F6 / F7: WebM → MP4 transcode path preserved; wait_for_text uses locator API; pytest marker remains AST-based.

Test plan

• pytest tests/test_demo_function.py tests/test_config.py (or full tests minus e2e as in CI)
• E2E kind: cli / VHS tests skip when vhs is not on PATH.