feat: Divi BlockTree – byte-exact parser/serializer with round-trip fixtures

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

tests/Fixtures/divi/synthetic-basic.html

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