fix(transformer): fail closed runtime DOM contracts

[chubes4]

Summary

• preserve root runtime data attributes only in same-element bounded islands instead of moving them onto RichText carriers
• retain native parent, sibling, ID, class, and source-equivalent inline contracts around bounded islands
• surface per-target native and fallback diagnostics, and promote missing required runtime targets to error-level compiler diagnostics

Validation

• composer --working-dir=php-transformer test
• 280 PHP transformer parity fixtures

Closes #1035. Follow-up to merged #1045 and #1059.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode general coding subagent inspected the post-merge findings, implemented same-element runtime data fallback boundaries and fail-closed compiler diagnostics, and ran the full PHP transformer verification. Chris Huber remains responsible for the changes.