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/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  ."))