Persist DKG extraction jobs before model calls

[branarakic-agent]

Summary

• persist source event identifiers before post-turn semantic extraction
• re-fetch signed evidence from the authenticated relay for every attempt
• promote successful extraction into the existing signed proposal outbox
• retry a bounded number of oldest pending jobs after later successful turns
• cap evidence to the integration contract's 16 signed source events

This closes the crash window between a successful Buzz response and creation of the signed DKG proposal. The human-facing turn remains successful even when memory finalization is pending.

Validation

• cargo fmt --all -- --check
• cargo clippy -p buzz-acp --lib -- -D warnings
• cargo test -p buzz-acp dkg_memory --lib (9 passed)