refactor(runtime): satisfy Sonar maintainability limits

[Brad-Edwards]

Summary

• extract participant crossing policy resolution from the oversized mediation function
• extract crossing snapshot construction and validation from the oversized decision function
• keep runtime manager and crossing modules below the 500-line repository and Sonar limits

This unblocks the Sonar quality gate on promotion PR #1049.

Verification

• 43 passed in focused participant crossing and opacity tests
• tools/check_repo_policy.py
• tools/check_requirement_governance.py (Ground Control unavailable; repository tool applied its documented skip)
• tools/verify_all.py: 5,979 unit tests passed, 56 integration tests passed, 92% coverage; contracts, static analysis, proof, and docs lanes passed