datalog: tokenizer raises on unexpected characters (256/256)

Bug: characters not recognised by any branch of `scan!` (`?`,
`!`, `#`, `@`, `&`, `|`, `\\`, `^`, etc.) were silently consumed
via `(else (advance! 1) (scan!))`. Programs with typos would
parse to a stripped version of themselves with no warning —
`?(X).` became `(X).` and produced confusing downstream errors.

Fix: the else branch now raises a clear "unexpected character"
error with the offending char and its position.

1 new tokenize test.

lib/datalog/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "lang": "datalog",
-  "total_passed": 255,
+  "total_passed": 256,
   "total_failed": 0,
-  "total": 255,
+  "total": 256,
   "suites": [
-    {"name":"tokenize","passed":29,"failed":0,"total":29},
+    {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
     {"name":"unify","passed":28,"failed":0,"total":28},
     {"name":"eval","passed":38,"failed":0,"total":38},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-10T21:13:14+00:00"
+  "generated": "2026-05-10T21:16:53+00:00"
 }