Preflight DOM box provider for fixture matrix

[chubes4]

Summary

• fail fast when fixture matrix DOM-box capture is requested without a provider command
• add a canonical DOM-box provider --preflight mode that verifies Playwright and Chromium before transforms run
• document the canonical fixture matrix DOM-box setup command and provider preflight

Validation

• homeboy --version -> 0.281.20+b9247da50759
• php figma-transformer/tests/contract/FixtureMatrixContract.php
• php -l figma-transformer/scripts/figma-fixture-matrix.php
• php -l figma-transformer/tests/contract/FixtureMatrixContract.php
• npm ci --prefix php-transformer/tools/visual-parity
• npm --prefix php-transformer/tools/visual-parity run install:browsers
• node php-transformer/tools/visual-parity/bin/dom-box-provider.mjs --preflight
• npm --prefix php-transformer/tools/visual-parity test

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.5 via OpenCode
• Used for: Inspected the fixture matrix harness and visual-parity provider setup, implemented the preflight hardening, added focused contract coverage, updated docs, and ran validation.