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/scoreboard.json

@@ -15,9 +15,10 @@
     "catalog": {"pass": 10, "fail": 0},
     "query": {"pass": 9, "fail": 0},
     "batch": {"pass": 10, "fail": 0},
+    "upcast": {"pass": 9, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 162,
+  "total_pass": 171,
   "total_fail": 0,
-  "total": 162
+  "total": 171
 }