Document the new DPF surfaces in one command set, and test the field, half-tree, and multipoint edges.

Co-authored-by: Cursor <cursoragent@cursor.com>

include/grotto/gadgets/binary/sgn.hpp

@@ -1,6 +1,6 @@
-/// @file grotto/gadgets/binary/sg.hpp
+/// @file grotto/gadgets/binary/sgn.hpp
 /// @author Ryan Henry <ryan.henry@ucalgary.ca>
-/// @brief
+/// @brief Cleartext reference for `sgn`.
 /// @copyright Copyright (c) 2019-2023 Ryan Henry and others
 /// @license Released under a GNU General Public v2.0 (GPLv2) license;
 ///          see [LICENSE.md](@ref GPLv2) for details.
@@ -20,7 +20,11 @@ namespace grotto
 namespace gadgets
 {
 
-struct sgn
+HEDLEY_DIAGNOSTIC_PUSH
+HEDLEY_DIAGNOSTIC_DISABLE_DEPRECATED
+
+
+struct HEDLEY_DEPRECATED_FOR(2026, grotto::exact_constant) sgn
 {
     template <typename T>
     T operator()(T x)
@@ -43,6 +47,8 @@ struct gadget_hints<sgn>
     static constexpr std::array<double, degree+1> canonical_polys[] = { -1, 0, 1 };
 };
 
+HEDLEY_DIAGNOSTIC_POP
+
 }  // namespace gadgets
 
 }  // namespace grotto