Add HTML artifact screenshot provider

[chubes4]

Summary

• Adds a Playwright-backed screenshot provider for static artifact-origin HTML pages.
• Writes deterministic PNG files under a caller-provided output directory and emits a JSON manifest on stdout.
• Documents env and flag usage next to the existing visual-parity DOM-box provider.

Fast path commands

• Install: npm ci --prefix php-transformer/tools/visual-parity && npm --prefix php-transformer/tools/visual-parity run install:browsers
• Capture via provider: HOMEBOY_SCREENSHOT_BASE_URL=http://127.0.0.1:7351 HOMEBOY_SCREENSHOT_PAGE_PATHS_JSON='["/workflow-bench/run/index.html"]' HOMEBOY_SCREENSHOT_OUTPUT_DIR=/path/to/screenshots node php-transformer/tools/visual-parity/bin/screenshot-provider.mjs --viewport=1440x900
• Expected PNG path: /path/to/screenshots/workflow-bench-run-index.html.png
• Zero-code fallback: npm --prefix php-transformer/tools/visual-parity exec playwright screenshot -- --viewport-size=1440,900 --full-page <artifact-origin-url> /path/to/screenshot.png

Testing

• npm test from php-transformer/tools/visual-parity

AI assistance

• AI assistance: Yes
• Tool(s): gpt-5.5 via OpenCode
• Used for: Researched the existing visual-parity/Homeboy artifact-origin contracts and drafted the small screenshot provider, docs, and smoke coverage.