fix: sync positions from decided cut to prevent gaps

[qj0r9j0vc2]

Summary

• Pass Cut to Primary when consensus decides, enabling position synchronization
• Add sync_positions_from_cut() to update last_seen_positions from decided CARs
• Prevents PositionGap errors that caused CAR attestation failures at height 3+

Root Cause

When consensus decided on a Cut, only height was passed to Primary. Validators that missed CARs during collection had stale position tracking, causing subsequent CARs to be rejected with PositionGap errors.

Test plan

• cargo check passes
• All 66 tests pass
• Devnet cluster test with 4 validators