Deduplicate generated block class attrs

[chubes4]

Summary

• Normalize block className attrs at the serializer boundary to strip generated Gutenberg classes and duplicate class tokens before comment attrs and saved HTML are produced.
• Preserve custom source classes such as hero-tagline while letting block support emit generated classes like has-text-color exactly once.
• Add contract coverage for the Coffee-style duplicate has-text-color paragraph mismatch through both full HTML transform and direct BlockFactory usage.

Evidence

• Lab blocker evidence run: 55267331-ad9c-4a71-9a41-be7cb9254c95.
• Before: 4 editor invalids; affected fixtures were 2-onepager-coffee, 3-artist-music (2 blocks), and 31-personal-cv-academic; snippets showed core/paragraph class mismatches like has-text-color has-text-color hero-tagline.
• After focused local fixture validation: 2-onepager-coffee, 3-artist-music, 31-personal-cv-academic, and 15-saas all report wp_block_validity=pass, 0 findings, 0 duplicate has-text-color, and 0 core/html fallback blocks.
• composer test:canonical
• composer test (includes 229 PHP transformer parity fixtures)

AI assistance

• AI assistance: Yes
• Tool(s): GPT-5.5 via OpenCode
• Used for: Investigated Lab artifacts, mapped the invalid paragraph mismatch to generated class leakage, implemented the serializer fix and regression coverage, and ran verification.