haskell: Phase 10 — math builtins (sqrt/floor/ceiling/round/truncate) (+6 tests, 28/28)

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

plans/haskell-completeness.md

@@ -160,7 +160,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   ints (`1.0e10` → `"10000000000"`) because our system can't distinguish
   `42` from `42.0` — both are SX numbers where `integer?` is true. Existing
   tests like `show 42 = "42"` rely on this rendering. Documented in `numerics.sx`._
-- [ ] Math builtins: `sqrt`, `floor`, `ceiling`, `round`, `truncate` — call
+- [x] Math builtins: `sqrt`, `floor`, `ceiling`, `round`, `truncate` — call
   the corresponding SX numeric primitives.
 - [ ] `Fractional` typeclass stub: `(/)`, `recip`, `fromRational`.
 - [ ] `Floating` typeclass stub: `pi`, `exp`, `log`, `sin`, `cos`, `(**)`
@@ -301,6 +301,12 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 10 math builtins (sqrt/floor/ceiling/round/truncate):
+- Inserted in the post-prelude `begin` block so they override the prelude's
+  identity stubs. `ceiling` is the only one needing a definition (SX doesn't
+  ship one — derived from `floor`). `sqrt`, `floor`, `round`, `truncate`
+  thunk through to SX primitives. 6 new tests in `numerics.sx` (now 28/28).
+
 **2026-05-07** — Phase 10 Float display through `hk-show-val`:
 - Added `hk-show-num` and `hk-show-float-sci` helpers in `eval.sx`. Number
   formatting: `integer?` → decimal (covers all whole-valued numbers, both ints