haskell: Phase 8 — showsPrec/showParen/shows/showString stubs (+7 tests, 7/7)

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

plans/haskell-completeness.md

@@ -107,7 +107,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   multi-constructor ADTs. Nested application arguments wrapped in parens:
   if `show arg` contains a space, emit `"(" ++ show arg ++ ")"`. _Records
   deferred — Phase 14._
-- [ ] `showsPrec` / `showParen` stubs so hand-written Show instances compile.
+- [x] `showsPrec` / `showParen` stubs so hand-written Show instances compile.
 - [ ] `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,
@@ -287,6 +287,18 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**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)`,
+  `showParen False p s = p s`, `showsPrec _ x s = show x ++ s`.
+- These let hand-written `Show` instances using `showsPrec`/`showParen` parse
+  and run; the precedence arg is ignored (we always defer to `show`'s built-in
+  precedence handling), but call shapes match Haskell 98 so user code compiles.
+- New `lib/haskell/tests/show.sx` (7 tests). The file is intended to grow to
+  ≥12 covering the full audit (Phase 8 ☐).
+- Function composition `.` is not yet bound; tests use manual composition via
+  let-binding. Address in a later iteration.
+
 **2026-05-06** — Phase 8 `deriving Show` nested constructor parens verified:
 - The Phase 8 audit's precedence-based `hk-show-prec` already does the right
   thing for `deriving Show`: each constructor arg is shown at prec 11, so any