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/compaction.sx

@@ -0,0 +1,43 @@
+; persist/compaction — once a snapshot subsumes a log prefix, those events are
+; dead weight: replay starts from the snapshot, so events with seq <= the
+; snapshot's seq are never folded again. Compaction checkpoints then truncates
+; that prefix. The seq counter is monotonic (backend high-water mark) so future
+; appends keep climbing — the surviving tail keeps its original seqs and replay
+; from the snapshot still equals a full replay of the pre-compaction log.
+; Policy is explicit: compact when the uncompacted tail reaches `every` events.
+; Requires: lib/persist/snapshot.sx, lib/persist/log.sx.
+
+; events accumulated since the last snapshot for name
+(define
+  persist/uncompacted
+  (fn
+    (b stream name seed)
+    (-
+      (persist/last-seq b stream)
+      (persist/project-seq (persist/snapshot-load b name seed)))))
+
+; policy: should we compact yet? tail since snapshot >= every
+(define
+  persist/should-compact?
+  (fn
+    (b stream name every seed)
+    (>= (persist/uncompacted b stream name seed) every)))
+
+; checkpoint then drop the snapshotted prefix; returns the new snapshot state
+(define
+  persist/compact
+  (fn
+    (b stream name step seed)
+    (let
+      ((state (persist/checkpoint b stream name step seed)))
+      (begin (persist/truncate b stream (persist/project-seq state)) state))))
+
+; compact only if the policy fires; always returns the current snapshot state
+(define
+  persist/maybe-compact
+  (fn
+    (b stream name step seed every)
+    (if
+      (persist/should-compact? b stream name every seed)
+      (persist/compact b stream name step seed)
+      (persist/snapshot-load b name seed))))