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

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

lib/haskell/tests/numerics.sx

@@ -98,4 +98,24 @@
   (hk-as-list (hk-deep-force (hk-run "main = map fromIntegral [1,2,3]")))
   (list 1 2 3))
 
+(hk-test
+  "toInteger 100 = 100 (identity)"
+  (hk-deep-force (hk-run "main = toInteger 100"))
+  100)
+
+(hk-test
+  "fromInteger 7 = 7 (identity)"
+  (hk-deep-force (hk-run "main = fromInteger 7"))
+  7)
+
+(hk-test
+  "toInteger / fromInteger round-trip"
+  (hk-deep-force (hk-run "main = fromInteger (toInteger 42)"))
+  42)
+
+(hk-test
+  "toInteger preserves negative"
+  (hk-deep-force (hk-run "main = toInteger (negate 13)"))
+  -13)
+
 {:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}