datalog: tokenizer raises on unterminated string + quoted atom (251/251)

Bug: read-quoted ran to EOF silently when the closing quote was
missing. The token's value was whatever ran-to-end string had been
accumulated; the parser later saw an unexpected EOF, but the error
message blamed the wrong location ("expected `)` got eof") and
hid the real problem.

Fix: read-quoted now raises with a message that distinguishes
strings from quoted atoms, including the position where the
opening quote was lost. The escape-sequence handling and proper
closing are unaffected.

2 new tokenize tests.

lib/datalog/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "lang": "datalog",
-  "total_passed": 249,
+  "total_passed": 251,
   "total_failed": 0,
-  "total": 249,
+  "total": 251,
   "suites": [
-    {"name":"tokenize","passed":27,"failed":0,"total":27},
+    {"name":"tokenize","passed":29,"failed":0,"total":29},
     {"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": "2026-05-10T20:59:17+00:00"
+  "generated": "2026-05-10T21:05:12+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,10 +1,10 @@
 # datalog scoreboard
 
-**249 / 249 passing** (0 failure(s)).
+**251 / 251 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
-| tokenize | 27 | 27 | ok |
+| tokenize | 29 | 29 | ok |
 | parse | 22 | 22 | ok |
 | unify | 28 | 28 | ok |
 | eval | 36 | 36 | ok |

lib/datalog/tests/tokenize.sx

@@ -118,6 +118,25 @@
         "block comment"
         (dl-tk-types (dl-tokenize "/* a\nb */ x."))
         (list "atom" "punct" "eof"))
+      ;; Unterminated string / quoted-atom must raise.
+      (dl-tk-test!
+        "unterminated string raises"
+        (let ((threw false))
+          (do
+            (guard (e (#t (set! threw true)))
+              (dl-tokenize "\"unclosed"))
+            threw))
+        true)
+
+      (dl-tk-test!
+        "unterminated quoted atom raises"
+        (let ((threw false))
+          (do
+            (guard (e (#t (set! threw true)))
+              (dl-tokenize "'unclosed"))
+            threw))
+        true)
+
       ;; Unterminated block comment must raise — previously it was
       ;; silently consumed to EOF.
       (dl-tk-test!

lib/datalog/tokenizer.sx

@@ -123,7 +123,11 @@
               (fn
                 ()
                 (cond
-                  ((>= pos src-len) nil)
+                  ((>= pos src-len)
+                    (error
+                      (str "Tokenizer: unterminated "
+                           (if (= quote-char "'") "quoted atom" "string")
+                           " (started near position " pos ")")))
                   ((= (cur) "\\")
                     (do
                       (advance! 1)