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/ocaml/lib/sx_primitives.ml

@@ -210,7 +210,10 @@ let () =
   register "acos" (fun args ->
     match args with [a] -> Number (Float.acos (as_number a)) | _ -> raise (Eval_error "acos: 1 arg"));
   register "atan" (fun args ->
-    match args with [a] -> Number (Float.atan (as_number a)) | _ -> raise (Eval_error "atan: 1 arg"));
+    match args with
+    | [a] -> Number (Float.atan (as_number a))
+    | [y; x] -> Number (Float.atan2 (as_number y) (as_number x))
+    | _ -> raise (Eval_error "atan: 1-2 args"));
   register "atan2" (fun args ->
     match args with [a; b] -> Number (Float.atan2 (as_number a) (as_number b))
     | _ -> raise (Eval_error "atan2: 2 args"));
@@ -320,6 +323,85 @@ let () =
     | [Number n] -> Integer (int_of_float (Float.round n))
     | [a] -> Integer (int_of_float (Float.round (as_number a)))
     | _ -> raise (Eval_error "inexact->exact: 1 arg"));
+  register "expt" (fun args ->
+    match args with
+    | [Integer a; Integer b] when b >= 0 ->
+      let rec ipow base e acc = if e = 0 then acc else ipow base (e - 1) (acc * base) in
+      Integer (ipow a b 1)
+    | [a; b] -> Number (Float.pow (as_number a) (as_number b))
+    | _ -> raise (Eval_error "expt: 2 args"));
+  register "quotient" (fun args ->
+    match args with
+    | [Integer a; Integer b] -> Integer (Int.div a b)
+    | [a; b] ->
+      let n = as_number a /. as_number b in
+      Integer (int_of_float (if n >= 0.0 then floor n else ceil n))
+    | _ -> raise (Eval_error "quotient: 2 args"));
+  let rec igcd a b = if b = 0 then a else igcd b (a mod b) in
+  register "gcd" (fun args ->
+    match args with
+    | [Integer a; Integer b] -> Integer (igcd (abs a) (abs b))
+    | [a; b] ->
+      let rec fgcd a b = if b = 0.0 then a else fgcd b (Float.rem a b) in
+      Number (fgcd (abs_float (as_number a)) (abs_float (as_number b)))
+    | _ -> raise (Eval_error "gcd: 2 args"));
+  register "lcm" (fun args ->
+    match args with
+    | [Integer a; Integer b] ->
+      let g = igcd (abs a) (abs b) in
+      if g = 0 then Integer 0 else Integer (abs a / g * abs b)
+    | [a; b] ->
+      let a = abs_float (as_number a) and b = abs_float (as_number b) in
+      let rec fgcd a b = if b = 0.0 then a else fgcd b (Float.rem a b) in
+      let g = fgcd a b in
+      if g = 0.0 then Number 0.0 else Number (a /. g *. b)
+    | _ -> raise (Eval_error "lcm: 2 args"));
+  register "number->string" (fun args ->
+    let digits = "0123456789abcdefghijklmnopqrstuvwxyz" in
+    let int_to_radix n r =
+      if n = 0 then "0"
+      else begin
+        let neg = n < 0 in
+        let buf = Buffer.create 16 in
+        let rec go n = if n > 0 then begin go (n / r); Buffer.add_char buf digits.[n mod r] end in
+        go (abs n);
+        (if neg then "-" else "") ^ Buffer.contents buf
+      end
+    in
+    match args with
+    | [Integer n] -> String (string_of_int n)
+    | [Number f] -> String (Printf.sprintf "%g" f)
+    | [Integer n; Integer r] ->
+      if r < 2 || r > 36 then raise (Eval_error "number->string: radix out of range");
+      String (int_to_radix n r)
+    | [Number f; Integer r] ->
+      if r < 2 || r > 36 then raise (Eval_error "number->string: radix out of range");
+      String (int_to_radix (int_of_float f) r)
+    | _ -> raise (Eval_error "number->string: 1-2 args"));
+  register "string->number" (fun args ->
+    match args with
+    | [String s] ->
+      (try Integer (int_of_string s)
+       with _ -> try Number (float_of_string s)
+       with _ -> Nil)
+    | [String s; Integer r] ->
+      (try
+        let neg = String.length s > 0 && s.[0] = '-' in
+        let start = if neg then 1 else 0 in
+        let n = ref 0 in
+        for i = start to String.length s - 1 do
+          let c = Char.code s.[i] in
+          let d = if c >= 48 && c <= 57 then c - 48
+                  else if c >= 97 && c <= 122 then c - 87
+                  else if c >= 65 && c <= 90  then c - 55
+                  else raise Exit
+          in
+          if d >= r then raise Exit;
+          n := !n * r + d
+        done;
+        Integer (if neg then - !n else !n)
+       with _ -> Nil)
+    | _ -> raise (Eval_error "string->number: 1-2 args"));
   register "parse-int" (fun args ->
     let parse_leading_int s =
       let len = String.length s in