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/tests/compaction.sx

@@ -0,0 +1,124 @@
+; Phase 3 — compaction: drop the snapshotted prefix; replay determinism holds.
+
+(define comp-count (fn (acc e) (+ acc 1)))
+
+(persist-test
+  "uncompacted counts events since snapshot"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/uncompacted b "s" "snap" 0)))
+  2)
+(persist-test
+  "should-compact? false below threshold"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/should-compact? b "s" "snap" 3 0)))
+  false)
+(persist-test
+  "should-compact? true at threshold"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/should-compact? b "s" "snap" 3 0)))
+  true)
+(persist-test
+  "compact truncates the snapshotted prefix"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/compact b "s" "snap" comp-count 0)
+      (persist/count b "s")))
+  0)
+(persist-test
+  "compact preserves logical last-seq"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/compact b "s" "snap" comp-count 0)
+      (persist/last-seq b "s")))
+  2)
+(persist-test
+  "append after compaction continues the seq"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/compact b "s" "snap" comp-count 0)
+      (persist/event-seq (persist/append b "s" "x" 0 {}))))
+  3)
+(persist-test
+  "replay after compaction == full count before compaction"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/compact b "s" "snap" comp-count 0)
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/project-value
+        (persist/replay b "s" "snap" comp-count 0))))
+  5)
+(persist-test
+  "determinism: post-compaction replay value equals uncompacted full replay"
+  (let
+    ((b (persist/open)) (c (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/append c "s" "x" 0 {})
+      (persist/append c "s" "x" 0 {})
+      (persist/append c "s" "x" 0 {})
+      (persist/compact b "s" "snap" comp-count 0)
+      (persist/append b "s" "x" 0 {})
+      (persist/append c "s" "x" 0 {})
+      (equal?
+        (persist/project-value
+          (persist/replay b "s" "snap" comp-count 0))
+        (persist/project-fold c "s" comp-count 0))))
+  true)
+(persist-test
+  "maybe-compact below threshold does not truncate"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/maybe-compact b "s" "snap" comp-count 0 5)
+      (persist/count b "s")))
+  1)
+(persist-test
+  "maybe-compact at threshold truncates"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/append b "s" "x" 0 {})
+      (persist/maybe-compact b "s" "snap" comp-count 0 2)
+      (persist/count b "s")))
+  0)
+(persist-test
+  "compact is idempotent on an empty tail"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append b "s" "x" 0 {})
+      (persist/compact b "s" "snap" comp-count 0)
+      (persist/project-value
+        (persist/compact b "s" "snap" comp-count 0))))
+  1)