haskell: Phase 8 — Read class stub (reads/readsPrec/read) (+3 tests, 10/10)

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

plans/haskell-completeness.md

@@ -108,7 +108,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   if `show arg` contains a space, emit `"(" ++ show arg ++ ")"`. _Records
   deferred — Phase 14._
 - [x] `showsPrec` / `showParen` stubs so hand-written Show instances compile.
-- [ ] `Read` class stub — just enough for `reads :: String -> [(a,String)]` to
+- [x] `Read` class stub — just enough for `reads :: String -> [(a,String)]` to
   type-check; no real parser needed yet.
 - [ ] Tests in `lib/haskell/tests/show.sx` (≥ 12 tests: show Int, show Bool,
   show Char, show String, show list, show tuple, show Maybe, show custom ADT,
@@ -287,6 +287,14 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 8 `Read` class stub (`reads`, `readsPrec`, `read`):
+- Three lines added to `hk-prelude-src`: `reads s = []`, `readsPrec _ s = reads s`,
+  `read s = fst (head (reads s))`. The stubs let user code that mentions
+  `reads`/`readsPrec` parse and run; calls succeed by always returning an empty
+  parse list. `read` will throw a pattern-match failure at runtime — fine until
+  Phase 9 `error` lands. No real parser needed per the plan.
+- 3 new tests in `tests/show.sx` (now 10/10).
+
 **2026-05-07** — Phase 8 `showsPrec` / `showParen` / `shows` / `showString` stubs:
 - Added 5 lines to `hk-prelude-src`. `shows x s = show x ++ s`,
   `showString prefix rest = prefix ++ rest`, `showParen True p s = "(" ++ p (")" ++ s)`,