datalog: tokenizer raises on unterminated block comment (249/249)

Bug: `/* unclosed` was silently consumed to EOF, swallowing any
Datalog code that followed inside the (never-closing) comment.
Programs would produce empty parses with no error.

Fix: skip-block-comment! now raises when it hits EOF without
finding `*/`. Error message includes the position where the
problem was first detected. Line comments (`%`) and properly
closed block comments (`/* ... */`) are unaffected.

1 new tokenize test verifies the error path.

lib/datalog/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "lang": "datalog",
-  "total_passed": 248,
+  "total_passed": 249,
   "total_failed": 0,
-  "total": 248,
+  "total": 249,
   "suites": [
-    {"name":"tokenize","passed":26,"failed":0,"total":26},
+    {"name":"tokenize","passed":27,"failed":0,"total":27},
     {"name":"parse","passed":22,"failed":0,"total":22},
     {"name":"unify","passed":28,"failed":0,"total":28},
     {"name":"eval","passed":36,"failed":0,"total":36},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":34,"failed":0,"total":34},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-10T20:55:23+00:00"
+  "generated": "2026-05-10T20:59:17+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,10 +1,10 @@
 # datalog scoreboard
 
-**248 / 248 passing** (0 failure(s)).
+**249 / 249 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
-| tokenize | 26 | 26 | ok |
+| tokenize | 27 | 27 | ok |
 | parse | 22 | 22 | ok |
 | unify | 28 | 28 | ok |
 | eval | 36 | 36 | ok |

lib/datalog/tests/tokenize.sx

@@ -118,6 +118,16 @@
         "block comment"
         (dl-tk-types (dl-tokenize "/* a\nb */ x."))
         (list "atom" "punct" "eof"))
+      ;; Unterminated block comment must raise — previously it was
+      ;; silently consumed to EOF.
+      (dl-tk-test!
+        "unterminated block comment raises"
+        (let ((threw false))
+          (do
+            (guard (e (#t (set! threw true)))
+              (dl-tokenize "/* unclosed comment"))
+            threw))
+        true)
       (dl-tk-test!
         "whitespace"
         (dl-tk-types (dl-tokenize "  foo  ,\t bar  ."))

lib/datalog/tokenizer.sx

@@ -63,7 +63,9 @@
         (fn
           ()
           (cond
-            ((>= pos src-len) nil)
+            ((>= pos src-len)
+              (error (str "Tokenizer: unterminated block comment "
+                          "(started at position " pos ")")))
             ((and (= (cur) "*") (< (+ pos 1) src-len) (= (dl-peek 1) "/"))
               (advance! 2))
             (else (do (advance! 1) (skip-block-comment!))))))