haskell: Phase 9 — undefined in prelude + lazy CAFs (+2 tests, 59/59)

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

plans/haskell-completeness.md

@@ -127,7 +127,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   `"Unhandled exception: <serialized>"` before any user handler sees them, so
   the tag has to live in a string prefix rather than as the head of a list.
   Catchers use `(index-of e "hk-error: ")` to detect.
-- [ ] `undefined :: a` = `error "Prelude.undefined"`.
+- [x] `undefined :: a` = `error "Prelude.undefined"`.
 - [ ] Partial functions emit proper error messages: `head []` →
   `"Prelude.head: empty list"`, `tail []` → `"Prelude.tail: empty list"`,
   `fromJust Nothing` → `"Maybe.fromJust: Nothing"`.
@@ -293,6 +293,18 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 9 `undefined = error "Prelude.undefined"` + lazy CAFs:
+- Added `undefined = error "Prelude.undefined"` to `hk-prelude-src`. Without
+  any other change this raised at prelude-load time because `hk-bind-decls!`
+  was eagerly evaluating zero-arity definitions (CAFs). Switched the CAF
+  binding from `(hk-eval body env)` to `(hk-mk-thunk body env)` — closer to
+  Haskell semantics: CAFs are not forced until first use.
+- The lazy-CAF change is a small but principled correctness fix; verified
+  no regressions across program-fib (uses `fibs`), program-sieve, primes,
+  infinite, seq, stdlib, class, do-io, quicksort.
+- 2 new tests in `tests/eval.sx` (raises with the right message; `undefined`
+  doesn't fire when not forced via `if True then 42 else undefined`). 59/59.
+
 **2026-05-07** — Phase 9 `error :: String -> a` raises with `hk-error:` prefix:
 - Pre-existing `error` builtin was raising `"*** Exception: <msg>"` (GHC
   console convention). Renamed prefix to `"hk-error: "` so the wrap-around