feat: add blocks only mode

[gokutheengineer]

No description provided.

[github-actions]

🤖 Claude Code Review

Status: Complete

Review Summary

This PR adds a BlocksOnly mode to subtree validation that allows nodes to skip processing peer-announced subtrees (via Kafka) while still processing subtrees from blocks. The implementation is clean and well-tested.

No issues found. The code follows project conventions and includes comprehensive test coverage:

• ✅ Configuration properly wired through settings with documentation
• ✅ Logic correctly placed after FSM state check
• ✅ Three test cases covering: blocks-only enabled, blocks-only disabled, and FSM state interaction
• ✅ Tests use appropriate patterns (testServer with mock injection, atomic.Bool for verification)
• ✅ Comments explain the "why" behind the feature