persist: compaction — drop snapshotted prefix, monotonic seq + 11 tests

Backend now tracks last-seq as a monotonic high-water mark (survives
truncation) and exposes :truncate-through. compaction.sx: persist/compact
checkpoints then drops events with seq <= snapshot seq; should-compact?/
maybe-compact give an explicit every-N policy. Determinism: post-compaction
replay value == uncompacted full replay. Phase 3 complete, 76/76.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/persist/scoreboard.md

@@ -11,4 +11,5 @@ _Generated by `lib/persist/conformance.sh`_
 | subscribe | 9 | 0 | 9 |
 | concurrency | 8 | 0 | 8 |
 | snapshot | 11 | 0 | 11 |
-| **Total** | **65** | **0** | **65** |
+| compaction | 11 | 0 | 11 |
+| **Total** | **76** | **0** | **76** |