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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(event log kv project subscribe concurrency snapshot)
+SUITES=(event log kv project subscribe concurrency snapshot compaction)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -34,6 +34,7 @@ run_suite() {
 (load "lib/persist/project.sx")
 (load "lib/persist/concurrency.sx")
 (load "lib/persist/snapshot.sx")
+(load "lib/persist/compaction.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)