haskell: Phase 12 — tests/set.sx (17/17, plan ≥15)

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

plans/haskell-completeness.md

@@ -210,9 +210,9 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   `toList`, `toAscList`, `size`, `null`, `union`, `intersection`, `difference`,
   `isSubsetOf`, `filter`, `map`, `foldr`, `foldl'`.
 - [x] Module wiring: `import Data.Set` / `import qualified Data.Set as Set`.
-- [ ] Unit tests in `lib/haskell/tests/set.sx` (≥ 15 tests: empty, insert,
+- [x] Unit tests in `lib/haskell/tests/set.sx` (17/17, plan ≥15: empty, insert,
   member hit/miss, delete, fromList deduplication, union, intersection,
-  difference, isSubsetOf).
+  difference, isSubsetOf, plus 4 end-to-end via `import qualified Data.Set`).
 - [ ] Conformance programs:
   - `uniquewords.hs` — unique words in a string using `Data.Set`.
   - `setops.hs` — set union/intersection/difference on integer sets;
@@ -307,6 +307,10 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 12 unit tests `tests/set.sx` (17/17):
+- 13 SX-level direct calls + 4 end-to-end via `import qualified Data.Set`.
+  Covers all the API + dedupe behavior. Suite is 17/17.
+
 **2026-05-07** — Phase 12 module wiring: `import Data.Set`:
 - New `hk-bind-data-set!` registers `Set.empty/singleton/insert/delete/
   member/size/null/union/intersection/difference/isSubsetOf` as Haskell