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"
 }

lib/datalog/scoreboard.md

@@ -1,10 +1,10 @@
 # datalog scoreboard
 
-**255 / 255 passing** (0 failure(s)).
+**256 / 256 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
-| tokenize | 29 | 29 | ok |
+| tokenize | 30 | 30 | ok |
 | parse | 22 | 22 | ok |
 | unify | 28 | 28 | ok |
 | eval | 38 | 38 | ok |

lib/datalog/tests/tokenize.sx

@@ -118,6 +118,18 @@
         "block comment"
         (dl-tk-types (dl-tokenize "/* a\nb */ x."))
         (list "atom" "punct" "eof"))
+      ;; Unexpected characters surface at tokenize time rather
+      ;; than being silently consumed (previously `?(X)` parsed as
+      ;; if the leading `?` weren't there).
+      (dl-tk-test!
+        "unexpected char raises"
+        (let ((threw false))
+          (do
+            (guard (e (#t (set! threw true)))
+              (dl-tokenize "?(X)"))
+            threw))
+        true)
+
       ;; Unterminated string / quoted-atom must raise.
       (dl-tk-test!
         "unterminated string raises"

lib/datalog/tokenizer.sx

@@ -254,7 +254,9 @@
                       (dl-emit! "op" "/" start)
                       (advance! 1)
                       (scan!)))
-                  (else (do (advance! 1) (scan!)))))))))
+                  (else (error
+                          (str "Tokenizer: unexpected character '" ch
+                               "' at position " start)))))))))
       (scan!)
       (dl-emit! "eof" nil pos)
       tokens)))