ocaml: phase 4 integer division semantics + Int module + max_int/min_int (+5 tests, 525 total)

Three things in this commit:

1. Integer / is now truncate-toward-zero on ints, IEEE on floats. The
   eval-op handler for '/' checks (number? + (= (round x) x)) on both
   sides; if both integral, applies host floor/ceil based on sign;
   otherwise falls through to host '/'.

2. Fixes Int.rem, which was returning 0 because (a - b * (a / b))
   was using float division: 17 - 5 * 3.4 = 0.0. Now Int.rem 17 5 = 2.

3. Int module fleshed out:
   max_int / min_int / zero / one / minus_one,
   succ / pred / neg, add / sub / mul / div / rem,
   equal, compare.

Also adds globals: max_int, min_int, abs_float, float_of_int,
int_of_float (the latter two are identity in our dynamic runtime).

  17 / 5         = 3
  -17 / 5        = -3   (trunc toward zero)
  Int.rem 17 5   = 2
  Int.compare 5 3 = 1

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 4 — integer `/` is now truncate-toward-zero on
+  ints, IEEE on floats. Both operands integral → host floor/ceil based
+  on sign; otherwise host `/`. Fixes `Int.rem` (which was returning 0
+  for `Int.rem 17 5` because `a / b` was producing a float). Also adds
+  Int.{max_int,min_int,zero,one,minus_one,succ,pred,neg,add,sub,mul,
+  div,rem,equal,compare} and global max_int/min_int/abs_float/
+  float_of_int/int_of_float (+5 tests, 525 total).
 - 2026-05-09 Phase 6 — Array.sort/stable_sort/fast_sort + sub +
   append + exists + for_all + mem (+5 tests, 520 total). All
   delegate to the corresponding List operation on the cell's