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"
 }