Normalize button labels for valid block markup

[chubes4]

Summary

• Normalize button labels by unwrapping block-level label tags and dropping empty decorative hidden spans before emitting core/button text.
• Add block validity detection for block-level markup inside serialized button links.
• Add contract/parity coverage for invalid button label markup.

Testing

• cd php-transformer && composer test
• Parity validity enumeration: invalid_html_transformer_fixture_count=0
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Drafted implementation and tests for generic Blocks Engine block-validity parity; Chris reviews and owns the change.