persist: event schema evolution via upcasters + 9 tests

upcast.sx: register a pure (event -> event) upcaster per type in an immutable
registry; read-upcast/project-upcast lift legacy events to the current shape on
read so projections see one shape (no version branching, no history rewrite).
upcast-data helper merges new :data fields. 171/171.

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

lib/persist/upcast.sx

@@ -0,0 +1,44 @@
+; persist/upcast — event schema evolution. An append-only log keeps events
+; forever, so old events have old shapes. Rather than migrate stored data (you
+; can't rewrite history) or branch every projection on version, register an
+; upcaster per event type: a pure (event -> event) that lifts an old event to
+; the current shape. Reads pass through the registry so projections see ONE
+; shape. The registry is an immutable dict the consumer threads (no global
+; mutable state). Requires: lib/persist/event.sx, lib/persist/log.sx.
+
+(define persist/upcasters (fn () {}))
+(define persist/register-upcaster (fn (reg type fn) (assoc reg type fn)))
+
+; apply the registered upcaster for an event's type, or pass it through unchanged
+(define
+  persist/upcast
+  (fn
+    (reg e)
+    (let ((f (get reg (persist/event-type e)))) (if f (f e) e))))
+
+; read a stream with every event lifted to current shape
+(define
+  persist/read-upcast
+  (fn
+    (b stream reg)
+    (map (fn (e) (persist/upcast reg e)) (persist/read b stream))))
+
+; project over upcasted events — projections never see a legacy shape
+(define
+  persist/project-upcast
+  (fn
+    (b stream reg step seed)
+    (reduce step seed (persist/read-upcast b stream reg))))
+
+; helper: upcast an event's :data by merging in/overriding fields, keeping the
+; record's stream/seq/type/at. Common upcaster body.
+(define
+  persist/upcast-data
+  (fn
+    (e new-data)
+    (persist/event
+      (persist/event-stream e)
+      (persist/event-seq e)
+      (persist/event-type e)
+      (persist/event-at e)
+      (merge (persist/event-data e) new-data))))