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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view cas catalog query batch recovery)
+SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view cas catalog query batch upcast recovery)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -41,6 +41,7 @@ run_suite() {
 (load "lib/persist/catalog.sx")
 (load "lib/persist/query.sx")
 (load "lib/persist/batch.sx")
+(load "lib/persist/upcast.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)