ocaml: phase 6 Printf %i/%u/%x/%X/%o + int_to_hex/octal host primitives (+5 tests, 533 total)

Three new host primitives in eval.sx:
  _int_to_hex_lower n  -> string of hex digits (lowercase)
  _int_to_hex_upper n  -> string of hex digits (uppercase)
  _int_to_octal    n   -> string of octal digits

Each builds the digit string by repeated floor(n / base) + mod,
prepending the digit at each step. Negative numbers prefix '-' so the
output round-trips through int_of_string with a sign.

Printf walker now fans out:
  %d, %i, %u  -> _string_of_int
  %f          -> _string_of_float
  %x          -> _int_to_hex_lower
  %X          -> _int_to_hex_upper
  %o          -> _int_to_octal
  %s, %c, %b  -> existing handling

  Printf.sprintf '%x' 255          = 'ff'
  Printf.sprintf '%X' 4096         = '1000'
  Printf.sprintf '%o' 8            = '10'
  Printf.sprintf '%x %X %o' 255 4096 8 = 'ff 1000 10'

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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 6 — Printf.sprintf adds %i, %u (aliases of %d),
+  %x (lowercase hex), %X (uppercase hex), %o (octal) (+5 tests, 533
+  total). New host primitives `_int_to_hex_lower`, `_int_to_hex_upper`,
+  `_int_to_octal` build the digit string by repeated host
+  `floor (/ n base)` + `mod`. The Printf walker fans out specs to the
+  right host helper. Examples: `%x` 255 = "ff", `%X` 4096 = "1000",
+  `%o` 8 = "10", multi: `%x %X %o` 255 4096 8 = "ff 1000 10".
 - 2026-05-09 Phase 6 — List.sort upgraded from O(n²) insertion sort
   to O(n log n) mergesort (+3 tests, 528 total). split + merge are
   inner functions of sort; tuple destructuring on the split result is