datalog: quoted 'atoms' tokenize as strings

Quoted atoms with uppercase- or underscore-leading names were
misclassified as variables. `p('Hello World').` flowed through the
tokenizer's "atom" branch and through the parser's string->symbol,
producing a symbol named "Hello World". dl-var? inspects the first
character — "H" is uppercase, so the fact was rejected as non-ground
("expected ground literal").

Tokenizer now emits "string" for any '...' quoted form. Quoted atoms
become opaque string constants — matching how Datalog idiomatically
treats them, and avoiding a per-symbol "quoted" marker that would
have rippled through unification and dl-var?. The trade-off is that
'a' and a are no longer the same value (string vs symbol); for
Datalog this is the safer default.

Updated the existing "quoted atom" tokenize test, added a regression
case for an uppercase-named quoted atom, and a parse-level test that
verifies the AST. Conformance 269/269.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/datalog/scoreboard.json

@@ -1,11 +1,11 @@
 {
   "lang": "datalog",
-  "total_passed": 267,
+  "total_passed": 269,
   "total_failed": 0,
-  "total": 267,
+  "total": 269,
   "suites": [
-    {"name":"tokenize","passed":30,"failed":0,"total":30},
-    {"name":"parse","passed":22,"failed":0,"total":22},
+    {"name":"tokenize","passed":31,"failed":0,"total":31},
+    {"name":"parse","passed":23,"failed":0,"total":23},
     {"name":"unify","passed":29,"failed":0,"total":29},
     {"name":"eval","passed":40,"failed":0,"total":40},
     {"name":"builtins","passed":26,"failed":0,"total":26},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T08:07:23+00:00"
+  "generated": "2026-05-11T08:39:03+00:00"
 }