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/tokenizer.sx

@@ -2,10 +2,13 @@
 ;;
 ;; Tokens: {:type T :value V :pos P}
 ;; Types:
-;;   "atom"    — lowercase-start ident or quoted 'atom'
+;;   "atom"    — lowercase-start bare identifier
 ;;   "var"     — uppercase-start or _-start ident (value is the name)
 ;;   "number"  — numeric literal (decoded to number)
-;;   "string"  — "..." string literal
+;;   "string"  — "..." string literal OR quoted 'atom' (treated as a
+;;               string value to avoid the var-vs-atom ambiguity that
+;;               would arise from a quoted atom whose name starts with
+;;               an uppercase letter or underscore)
 ;;   "punct"   — ( ) , .
 ;;   "op"      — :- ?- <= >= != < > = + - * /
 ;;   "eof"
@@ -192,7 +195,11 @@
                       (dl-emit! "number" (read-number start) start)
                       (scan!)))
                   ((= ch "'")
-                    (do (dl-emit! "atom" (read-quoted "'") start) (scan!)))
+                    ;; Quoted 'atoms' tokenize as strings so a name
+                    ;; like 'Hello World' doesn't get misclassified
+                    ;; as a variable by dl-var? (which inspects the
+                    ;; symbol's first character).
+                    (do (dl-emit! "string" (read-quoted "'") start) (scan!)))
                   ((= ch "\"")
                     (do (dl-emit! "string" (read-quoted "\"") start) (scan!)))
                   ((dl-lower? ch)