haskell: Phase 10 — toInteger/fromInteger verified as prelude identities (+4 tests, 18/18)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-completeness.md

@@ -152,7 +152,8 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   (all numbers share one SX type); register as a builtin no-op with the correct
   typeclass signature. _Already in `hk-prelude-src` as `fromIntegral x = x`;
   verified with new tests in `numerics.sx`._
-- [ ] `toInteger`, `fromInteger` — same treatment.
+- [x] `toInteger`, `fromInteger` — same treatment. _Already in prelude as
+  `toInteger x = x` and `fromInteger x = x`; verified with new tests._
 - [ ] Float/Double literals round-trip through `hk-show-val`:
   `show 3.14 = "3.14"`, `show 1.0e10 = "1.0e10"`.
 - [ ] Math builtins: `sqrt`, `floor`, `ceiling`, `round`, `truncate` — call
@@ -296,6 +297,11 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 10 `toInteger` / `fromInteger` verified (prelude identities):
+- Both already declared as `x = x` in `hk-prelude-src`. Added 4 tests in
+  `numerics.sx` (positive, identity round-trip, negative-via-negate, fromInteger
+  smoke). Suite now 18/18.
+
 **2026-05-07** — Phase 10 `fromIntegral` verified (already an identity in prelude):
 - Pre-existing `fromIntegral x = x` line in `hk-prelude-src` was already
   correct — all numbers share one SX type, so the identity implementation is