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>

plans/datalog-on-sx.md

@@ -15,7 +15,7 @@ for rose-ash data (e.g. federation graph, content relationships).
 
 ## Status (rolling)
 
-`bash lib/datalog/conformance.sh` → **267/267 across 11 suites**
+`bash lib/datalog/conformance.sh` → **269/269 across 11 suites**
 (tokenize, parse, unify, eval, builtins, semi_naive, negation, aggregates,
 api, magic, demo). Source is ~3100 LOC, tests ~2900 LOC, public API
 documented in `lib/datalog/datalog.sx`.
@@ -320,6 +320,18 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-11 — Quoted atoms with uppercase-or-underscore-leading
+  names were misclassified as variables. `p('Hello World').` ran
+  through the tokenizer's `"atom"` branch and through the parser's
+  `string->symbol`, producing a symbol named "Hello World". dl-var?
+  checks the first character — "H" is uppercase, so the fact was
+  rejected as non-ground. Fix: tokenizer emits `"string"` for any
+  `'...'` quoted form, so quoted atoms become opaque string constants
+  (matching how Datalog idiomatically treats them — the alternative
+  was a per-symbol "quoted" marker which would have rippled through
+  unification and dl-var?). Updated the existing tokenize test and
+  added one for `'Hello'`; also added a parse-level regression. 269/269.
+
 - 2026-05-11 — Type-mixed comparisons were silently inconsistent:
   `<(X, 5)` with `X` bound to a string returned `()` (no result, no
   error), while `X` bound to a symbol raised "Expected number, got