vm-ext: rational cleanup — (/ int int) returns float per spec, fix number?/exact? on Rational

The OP_DIV/numeric-tower work on this branch made the OCaml `/` primitive
return an exact Rational for (/ int int) (e.g. (/ 5 2)=5/2), diverging from
the canonical spec ("/ always returns inexact float"), the test-rationals.sx
header ("in the JS host, (/ int int) returns float — backward-compatible"),
and the JS host itself. That leaked rationals into arithmetic results and
rendered CSS (tw-opacity emitted `opacity:1/20` instead of `0.05`).

Decision (with the user): keep exact rationals as an explicit opt-in
(literals 1/3, make-rational) but bring `/` back into spec/host parity —
the isomorphic SSR↔hydration invariant requires both hosts to agree, and
JS has no native rational type.

- sx_primitives.ml `/`: (/ int int) → integer when exactly divisible, else
  inexact float; a Rational operand still yields an exact rational (matches
  test-numeric-tower: (/ 6 2)=3, (/ 1 4)=0.25, (/ 5 2)=2.5, (/ 1/2 2)=1/4).
- sx_primitives.ml number? / exact?: recognise the Rational type (real bugs —
  test-rationals asserts (number? 1/3) and (exact? 1/3); inexact?/float?
  already returned false for Rational, correct).
- sx_vm.ml OP_DIV: comment updated (it delegates to the now-float `/`).
- test-rationals.sx: fix typo in "rational * float = float" — used int 2,
  should be 2.0 (1/2 * 2 = 1 exact, not a float; name + siblings use floats).

OCaml conformance 4834→4863 (+29 fixed, zero new failures); rationals,
numeric-tower, arithmetic, tw-opacity suites all 0 failures. Remaining run_tests
failures are the pre-existing environmental hyperscript (host-call-fn) set.
JS host already handles number?/exact? on rationals and float `/`; its
remaining float?/contagion failures are a separate pre-existing limitation
(JS has no distinct float type), out of scope here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

spec/tests/test-rationals.sx

@@ -101,7 +101,7 @@
   "rationals:float-contagion"
   (deftest "rational + float = float" (assert (float? (+ 1/3 0.5))))
   (deftest "float + rational = float" (assert (float? (+ 0.5 1/3))))
-  (deftest "rational * float = float" (assert (float? (* 1/2 2))))
+  (deftest "rational * float = float" (assert (float? (* 1/2 2.0))))
   (deftest "rational - float = float" (assert (float? (- 1/2 0.1)))))
 
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