haskell: Phase 11 — Data.Map BST skeleton (Adams weight-balanced)

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

plans/haskell-completeness.md

@@ -179,7 +179,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 ### Phase 11 — Data.Map
 
-- [ ] Implement a weight-balanced BST in pure SX in `lib/haskell/map.sx`.
+- [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`,
@@ -304,6 +304,13 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**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
+  + accessors + the four rotations (single-l, single-r, double-l, double-r)
+  + `hk-map-balance` smart constructor that picks the rotation. Spot-checked
+  with eval calls; user-facing operations (insert/lookup/etc.) come next.
+
 **2026-05-07** — Phase 10 conformance: statistics.hs (5/5) + newton.hs (5/5) → Phase 10 complete:
 - `program-statistics.sx`: mean / variance / stdDev on a [Double], exercising
   `sum`, `map`, `fromIntegral`, `/`, `sqrt`. 5/5.