ocaml: phase 5.1 base_n.ml baseline (int to base-N string, length sum = 17)

36-character digit alphabet '0..9A..Z' supports any base 2..36. Loop
divides the magnitude by base and prepends the digit:

  while !m > 0 do
    acc := String.make 1 digits.[!m mod base] ^ !acc;
    m := !m / base
  done

Special-cases n = 0 -> '0' and prepends '-' for negatives.

Test cases (length, since the strings differ in alphabet):
  255 hex      'FF'              2
  1024 binary  '10000000000'    11
  100 dec      '100'             3
  0 any base   '0'               1
  sum                            17

Combines digits.[i] (string indexing) + String.make 1 ch + String
concatenation in a loop.

61 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,16 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — base_n.ml baseline (int -> base-N string,
+  length sum 2+11+3+1 = 17). 36-character digit alphabet supports up
+  to base 36. Loop divides quotient by base, prepends digit. Tests:
+    255 hex     "FF"             2
+    1024 binary "10000000000"   11
+    100 dec     "100"            3
+    0 any base  "0"              1
+  Sum 17. Combines digits.[!m mod base] (string indexing) +
+  String.make 1 ch (1-char string from 1-char string in our model) +
+  String concatenation in a loop. 61 baseline programs total.
 - 2026-05-09 Phase 5.1 — prime_factors.ml baseline (trial-division
   factorisation, sum of factors of 360 = 17). Three refs threading
   through a while loop: m holds the remaining quotient, d the