spec: math completeness — trig, quotient, gcd/lcm, radix number<->string

Phase 15 implementation:
- spec/primitives.sx: stdlib.math module — sin/cos/tan/asin/acos/atan/exp/log/expt/quotient/gcd/lcm/number->string/string->number (13 primitives)
- JS platform: stdlib.math module; strict string->number parsing (rejects partial matches like "fg" in base 16)
- OCaml: expt, quotient, gcd, lcm, number->string (radix), string->number (radix); atan updated to accept optional 2nd arg (atan2 form)
- spec/tests/test-math.sx: 44 tests — trig/inverse trig, expt, quotient semantics, gcd/lcm, radix formatting/parsing, tower integration
- JS: 2311/4801 (+2 net); OCaml: 4547/5629 (+1 net); zero regressions in math area

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

spec/primitives.sx

@@ -948,4 +948,90 @@
   :returns "boolean"
   :doc "True if a char is immediately available on the port.")
 
+(define-module :stdlib.math)
+
+(define-primitive
+  "sin"
+  :params ((x :as number))
+  :returns "float"
+  :doc "Sine of x (radians).")
+
+(define-primitive
+  "cos"
+  :params ((x :as number))
+  :returns "float"
+  :doc "Cosine of x (radians).")
+
+(define-primitive
+  "tan"
+  :params ((x :as number))
+  :returns "float"
+  :doc "Tangent of x (radians).")
+
+(define-primitive
+  "asin"
+  :params ((x :as number))
+  :returns "float"
+  :doc "Arc sine of x; result in radians.")
+
+(define-primitive
+  "acos"
+  :params ((x :as number))
+  :returns "float"
+  :doc "Arc cosine of x; result in radians.")
+
+(define-primitive
+  "atan"
+  :params ((x :as number) &rest (y :as number))
+  :returns "float"
+  :doc "Arc tangent. (atan x) → radians in (-π/2, π/2). (atan y x) → atan2(y, x).")
+
+(define-primitive
+  "exp"
+  :params ((x :as number))
+  :returns "float"
+  :doc "e raised to the power x.")
+
+(define-primitive
+  "log"
+  :params ((x :as number))
+  :returns "float"
+  :doc "Natural logarithm of x.")
+
+(define-primitive
+  "expt"
+  :params ((base :as number) (exp :as number))
+  :returns "number"
+  :doc "base raised to the power exp. Alias: pow.")
+
+(define-primitive
+  "quotient"
+  :params ((a :as number) (b :as number))
+  :returns "integer"
+  :doc "Integer quotient: truncate(a / b) toward zero. Sign follows dividend.")
+
+(define-primitive
+  "gcd"
+  :params ((a :as number) (b :as number))
+  :returns "integer"
+  :doc "Greatest common divisor of a and b.")
+
+(define-primitive
+  "lcm"
+  :params ((a :as number) (b :as number))
+  :returns "integer"
+  :doc "Least common multiple of a and b.")
+
+(define-primitive
+  "number->string"
+  :params ((n :as number) &rest (radix :as number))
+  :returns "string"
+  :doc "Convert number n to string. Optional radix (default 10). E.g. (number->string 255 16) → \"ff\".")
+
+(define-primitive
+  "string->number"
+  :params ((s :as string) &rest (radix :as number))
+  :returns "any"
+  :doc "Parse string s as a number. Optional radix (default 10). Returns nil on failure.")
+
 (define-module :stdlib.hash-table)