spec: rational numbers — 1/3 literals, arithmetic, numeric tower integration

SxRational type in OCaml (Rational of int * int, stored reduced, denom>0)
and JS (SxRational class with _rational marker). n/d reader syntax in
spec/parser.sx. Arithmetic contagion: int op rational → rational, rational
op float → float. JS keeps int/int → float for CSS backward compatibility.
OCaml as_number + safe_eq extended for cross-type rational equality so
(= 2.5 5/2) → true. 62 tests in test-rationals.sx, all pass.
JS: 2232 passed. OCaml: 4532 passed (+11 vs pre-fix baseline).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/primitives.sx

@@ -1034,4 +1034,30 @@
   :returns "any"
   :doc "Parse string s as a number. Optional radix (default 10). Returns nil on failure.")
 
+(define-module :stdlib.rational)
+
+(define-primitive
+  "make-rational"
+  :params (n d)
+  :returns "rational"
+  :doc "Rational n/d, auto-reduced by GCD. Error if d=0.")
+
+(define-primitive
+  "rational?"
+  :params (v)
+  :returns "boolean"
+  :doc "True if v is a rational number.")
+
+(define-primitive
+  "numerator"
+  :params ((r :as rational))
+  :returns "integer"
+  :doc "Numerator of rational r (after reduction).")
+
+(define-primitive
+  "denominator"
+  :params ((r :as rational))
+  :returns "integer"
+  :doc "Denominator of rational r (after reduction, always positive).")
+
 (define-module :stdlib.hash-table)