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>

hosts/javascript/platform.py

@@ -1427,6 +1427,49 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
     if (a === 0) return 0;
     return 32 - Math.clz32(Math.abs(a));
   };
+''',
+    "stdlib.math": '''
+  // stdlib.math
+  PRIMITIVES["sin"]  = Math.sin;
+  PRIMITIVES["cos"]  = Math.cos;
+  PRIMITIVES["tan"]  = Math.tan;
+  PRIMITIVES["asin"] = Math.asin;
+  PRIMITIVES["acos"] = Math.acos;
+  PRIMITIVES["atan"] = function(y, x) { return arguments.length >= 2 ? Math.atan2(y, x) : Math.atan(y); };
+  PRIMITIVES["exp"]  = Math.exp;
+  PRIMITIVES["log"]  = Math.log;
+  PRIMITIVES["expt"] = Math.pow;
+  PRIMITIVES["quotient"] = function(a, b) { return Math.trunc(a / b); };
+  PRIMITIVES["gcd"] = function(a, b) {
+    a = Math.abs(a); b = Math.abs(b);
+    while (b) { var t = b; b = a % b; a = t; }
+    return a;
+  };
+  PRIMITIVES["lcm"] = function(a, b) {
+    var g = PRIMITIVES["gcd"](Math.abs(a), Math.abs(b));
+    return g === 0 ? 0 : Math.abs(a / g * b);
+  };
+  PRIMITIVES["number->string"] = function(n, r) {
+    if (r === undefined || r === null) return String(n);
+    return Math.floor(n).toString(r);
+  };
+  PRIMITIVES["string->number"] = function(s, r) {
+    s = String(s);
+    if (r !== undefined && r !== null) {
+      var radix = r | 0;
+      var valid = "0123456789abcdefghijklmnopqrstuvwxyz".slice(0, radix);
+      var norm = s.toLowerCase();
+      var start = norm[0] === '-' ? 1 : 0;
+      if (norm.length <= start) return NIL;
+      for (var i = start; i < norm.length; i++) {
+        if (valid.indexOf(norm[i]) === -1) return NIL;
+      }
+      return parseInt(s, radix);
+    }
+    if (s === '') return NIL;
+    var n = Number(s);
+    return isNaN(n) ? NIL : n;
+  };
 ''',
     "stdlib.hash-table": '''
   // stdlib.hash-table