content: snapshot cache over op-log replay (snapshot.sx) + 20 tests (338/338)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/content-on-sx.md

@@ -19,7 +19,7 @@ injected adapter, not core.
 
 ## Status (rolling)
 
-`bash lib/content/conformance.sh` → **318/318** (Phases 1–4 COMPLETE + extensions: HTML/SX escaping, Markdown render+import, durable CRDT replication, validation)
+`bash lib/content/conformance.sh` → **338/338** (Phases 1–4 COMPLETE + extensions: HTML/SX escaping, Markdown render+import, durable CRDT replication, validation, snapshot cache)
 
 ## Ground rules
 
@@ -82,9 +82,18 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] durable CRDT replication (`crdt-store.sx`: ops on persist, replay + converge)
 - [x] document validation (`validate.sx`: ids, per-type fields, duplicate ids)
 - [x] Markdown import adapter (`md-import.sx`: text → blocks, round-trips export)
+- [x] snapshot cache over replay (`snapshot.sx`: cache-not-primary, transparent)
 
 ## Progress log
 
+- 2026-06-07 — Extension: snapshot cache over op-log replay (`snapshot.sx`).
+  Snapshots are a cache, never primary state — the log stays the source of truth.
+  `content/snapshot!` stores a materialised head at a seq in the persist KV;
+  `content/head-cached` / `content/at-cached` start from the nearest snapshot and
+  replay only the tail, returning a document IDENTICAL to a full replay (tests
+  assert transparency before/after snapshot, across versions, and after
+  drop-snapshot fallback). `content/has-snapshot?` / `snapshot-seq` /
+  `drop-snapshot!`. 20 tests; suite 338/338.
 - 2026-06-07 — Extension: Markdown import adapter (`md-import.sx`), inverse of
   asMarkdown. Line-based parser: ATX headings, fenced code (```lang), blockquotes,
   unordered/ordered lists (grouping consecutive items), thematic breaks,