fix(ledger): writer guard re-checks ownership and self-demotes on stale heartbeat

test(ledger): prove compaction crash safety with fault injection and against the real bucket

feat(ledger): add heartbeat-based writer guard

fix(ledger): heal truncated ledger, validate events on write, reconcile pending

feat(ledger): add Ledger service with event write path and compaction

feat(ledger): add event schema, jsonl encoding and fold