haskell: Phase 11 — Data.Map core operations (singleton/insert/lookup/delete/member/null)

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

plans/haskell-completeness.md

@@ -182,7 +182,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 - [x] Implement a weight-balanced BST in pure SX in `lib/haskell/map.sx`.
   Internal node representation: `("Map-Node" key val left right size)`.
   Leaf: `("Map-Empty")`.
-- [ ] Core operations: `empty`, `singleton`, `insert`, `lookup`, `delete`,
+- [x] Core operations: `empty`, `singleton`, `insert`, `lookup`, `delete`,
   `member`, `size`, `null`.
 - [ ] Bulk operations: `fromList`, `toList`, `toAscList`, `keys`, `elems`.
 - [ ] Combining: `unionWith`, `intersectionWith`, `difference`.
@@ -304,6 +304,15 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 11 core operations on `Data.Map` BST:
+- Added `hk-map-singleton`, `hk-map-insert`, `hk-map-lookup`, `hk-map-delete`,
+  `hk-map-member`, `hk-map-null`. Insert recurses with `hk-map-balance` to
+  maintain weight invariants. Lookup returns `("Just" v)` / `("Nothing")` —
+  matches Haskell ADT layout. Delete uses a `hk-map-glue` helper that picks
+  the larger subtree and pulls its extreme element to the root, preserving
+  balance without imperative state. Spot-checked: insert+lookup hit/miss,
+  member, delete root with successor pulled from right.
+
 **2026-05-07** — Phase 11 BST skeleton in `lib/haskell/map.sx`:
 - Adams-style weight-balanced tree: node = `("Map-Node" k v l r size)`,
   empty = `("Map-Empty")`. delta=3 / gamma=2 ratios. Implemented constructors