Preserve button shadows and tighten button diagnostics

[chubes4]

Summary

• Preserve source button box-shadow as canonical style.shadow on core/button and serialize it into the saved button control.
• Tighten button/menu visual diagnostics so pseudo-state CSS does not contaminate resting-state comparisons, CSS custom properties resolve before comparison, core button width classes count as width parity, and shadow/glow mismatches are classified.
• Add PHP coverage for button shadow emission and diagnostic regressions.

Verification

• php tests/unit/button-style-resolver.php
• php tests/unit/button-visual-probe-diagnostics.php
• php tests/unit/block-style-support-conversion.php
• composer test:unit
• Scoped SSI matrix run with local runner, this branch as Blocks Engine, this branch php-transformer override, fixture root limited to 15-saas and 3-artist-music, output at /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/be-saas-yellow-button-after.json, artifact root at /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/be-saas-yellow-button-after-artifacts.

Results

• SaaS visual mismatch improved from 42.08% in the clean postmerge scorecard to 40.17% in the scoped verification.
• Artist front-page visual mismatch in the scoped verification: 9.68%.
• Remaining scoped gate failures are broad visual mismatch/static-import issues, not amber CTA fill/text/shadow parity.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.5 via OpenCode
• Used for: Investigating button parity diagnostics, implementing PHP transformer/diagnostic changes, adding tests, and running verification.