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/log.sx

@@ -1,14 +1,18 @@
-; persist/log — the log facet: append-only event streams. seq is assigned
-; sequentially per stream (1-based). Reads return events oldest-first.
+; persist/log — the log facet: append-only event streams. seq is assigned from
+; a monotonic per-stream high-water mark (1-based) held by the backend, so it
+; keeps climbing even after the log prefix is compacted away. Reads return the
+; events currently stored, oldest-first.
 ; Requires: lib/persist/event.sx, lib/persist/backend.sx.
 
-; current length of a stream
+; logical last seq assigned in a stream (0 if none) — survives compaction
 (define
-  persist/stream-len
-  (fn (b stream) (len (persist/backend-read b stream))))
+  persist/last-seq
+  (fn (b stream) (persist/backend-last-seq b stream)))
 
-; last seq in a stream (0 if empty)
-(define persist/last-seq (fn (b stream) (persist/stream-len b stream)))
+; number of events physically stored in a stream (shrinks on compaction)
+(define
+  persist/count
+  (fn (b stream) (len (persist/backend-read b stream))))
 
 ; append an event, auto-assigning the next seq. Returns the stored event.
 (define
@@ -21,7 +25,7 @@
         ((ev (persist/event stream seq type at data)))
         (begin (persist/backend-append b stream ev) ev)))))
 
-; read all events in a stream, oldest-first
+; read all events currently stored in a stream, oldest-first
 (define persist/read (fn (b stream) (persist/backend-read b stream)))
 
 ; read events with seq >= from
@@ -33,5 +37,7 @@
       (fn (e) (>= (persist/event-seq e) from))
       (persist/read b stream))))
 
-; number of events in a stream
-(define persist/count (fn (b stream) (persist/stream-len b stream)))
+; drop events with seq <= n (compaction); the seq counter is untouched
+(define
+  persist/truncate
+  (fn (b stream n) (persist/backend-truncate b stream n)))