haskell: reject untypeable programs — hk-typecheck + hk-run-typed (+9 tests, 464/464)

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

plans/haskell-on-sx.md

@@ -93,7 +93,7 @@ Key mappings:
 ### Phase 4 — Hindley-Milner inference
 - [x] Algorithm W: unification + type schemes + generalisation + instantiation
 - [x] Report type errors with meaningful positions
-- [ ] Reject untypeable programs that phase 3 was accepting
+- [x] Reject untypeable programs that phase 3 was accepting
 - [ ] Type-sig checking: user writes `f :: Int -> Int`; verify
 - [ ] Let-polymorphism
 - [ ] Unit tests: inference for 50+ expressions
@@ -114,6 +114,17 @@ Key mappings:
 
 _Newest first._
 
+- **2026-05-05** — Phase 4 reject untypeable programs. `hk-typecheck` runs
+  `hk-infer-prog` on a program AST and raises the first type error found.
+  `hk-run-typed` is a drop-in for `hk-run` that gates evaluation on a
+  successful type check. `hk-infer-decl` now returns a 4th element (raw type
+  value); `hk-infer-prog` propagates inferred types into the running type env
+  so multi-function programs (`f x = x+1\ng y = f y+2`) infer correctly.
+  test.sh extended to load infer.sx for `*typecheck*` files.
+  9 new tests in `tests/typecheck.sx`: 4 valid programs pass through, 5
+  invalid programs are rejected (Int+Bool, non-Bool if condition, unbound var,
+  apply non-function). 464/464 green.
+
 - **2026-05-05** — Phase 4 type error reporting. `hk-expr->brief` converts any AST
   node to a short human-readable string for error messages (handles var/con/int/float/
   str/char/bool/app/op/if/let/lambda/tuple/list/loc). `loc` nodes in `hk-w` delegate