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.json

@@ -6,9 +6,10 @@
     "project": {"pass": 9, "fail": 0},
     "subscribe": {"pass": 9, "fail": 0},
     "concurrency": {"pass": 8, "fail": 0},
-    "snapshot": {"pass": 11, "fail": 0}
+    "snapshot": {"pass": 11, "fail": 0},
+    "compaction": {"pass": 11, "fail": 0}
   },
-  "total_pass": 65,
+  "total_pass": 76,
   "total_fail": 0,
-  "total": 65
+  "total": 76
 }