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/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!