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"
 }