Normalize button diagnostic style shorthands

[chubes4]

Summary

• Normalize border shorthand/longhand forms before button visual probe comparison.
• Normalize wrapper chrome comparison through the same comparable style groups so equivalent padding/border syntax does not produce false residual causes.
• Add unit coverage for equivalent border and wrapper shorthand/longhand diagnostics.

Diagnostic evidence

• Before, current trunk SaaS button probe reported 6 style deltas and causes: button_border_mismatch=4, button_text_color_mismatch=2, button_wrapper_chrome_mismatch=8.
• After, SaaS button probe reports 2 style deltas and causes: button_text_color_mismatch=2, button_wrapper_chrome_mismatch=3.
• Artist remains unchanged: real button_default_style_leakage=9, button_border_missing=9, button_text_color_mismatch=10, button_padding_missing=4, button_fill_missing=1, button_wrapper_chrome_mismatch=3.

Verification

• composer test
• php tools/static-parity/run.php --fixture 15-saas --json
• php tools/static-parity/run.php --fixture 3-artist-music --json

AI assistance

• AI assistance: Yes
• Tool(s): GPT-5.5 via OpenCode
• Used for: Investigated residual SaaS/Artist button diagnostics, implemented the comparator normalization, added tests, and ran verification.