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.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 |