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>

plans/persist-on-sx.md

@@ -42,7 +42,7 @@ read models (feeds, indices, audit logs) update incrementally.
 
 ## Status (rolling)
 
-`bash lib/persist/conformance.sh` → **54/54** (Phases 1–2 done)
+`bash lib/persist/conformance.sh` → **76/76** (Phases 1–3 done)
 
 ## Ground rules
 
@@ -100,7 +100,7 @@ lib/persist/backend.sx              lib/persist/api.sx
 
 ## Phase 3 — Snapshots + replay
 - [x] `snapshot.sx` — checkpoint a projection; replay = snapshot + tail
-- [ ] compaction policy; replay-determinism tests
+- [x] compaction policy; replay-determinism tests
 
 ## Phase 4 — Durable backends via kernel IO
 - [ ] file/log backend driven through `perform` (IO-suspension boundary)
@@ -113,6 +113,15 @@ feed/-log, flow store, mod/audit, search index, acl grants, identity sessions al
 become `persist` log or kv. Track each migration in that subsystem's plan.
 
 ## Progress log
+- **Phase 3b (76/76) — Phase 3 complete.** Backend refactor: `last-seq` is now
+  a monotonic per-stream high-water mark (backend `seqs` dict), not physical
+  length, so a compacted log keeps assigning climbing seqs. Added backend
+  `:truncate-through` + `persist/truncate`. `compaction.sx` — `persist/compact`
+  checkpoints then drops events with seq <= snapshot seq; `should-compact?`/
+  `maybe-compact` give an explicit "compact every N tail events" policy. 11
+  tests: post-compaction replay value == uncompacted full replay (determinism),
+  seq continuity after truncation, idempotence. `persist/count` = physical
+  stored count (shrinks on compaction) vs `persist/last-seq` = logical.
 - **Phase 3a (65/65).** `snapshot.sx` — a snapshot is a projection state
   `{:value :seq}` stored in the kv facet under `snapshot/<name>`.
   `persist/checkpoint` replays + saves; `persist/replay` = snapshot + tail.