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/tests/parse.sx

@@ -106,6 +106,13 @@
         "string arg"
         (dl-parse "label(x, \"hi\").")
         (list {:body (list) :head (list (quote label) (quote x) "hi")}))
+      ;; Quoted 'atoms' parse as strings — a uppercase-starting name
+      ;; in quotes used to misclassify as a variable and reject the
+      ;; fact as non-ground.
+      (dl-pt-test!
+        "quoted atom arg parses as string"
+        (dl-parse "p('Hello World').")
+        (list {:body (list) :head (list (quote p) "Hello World")}))
       (dl-pt-test!
         "comparison literal"
         (dl-parse "p(X) :- <(X, 5).")