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/dpf/packed_lane.hpp

@@ -41,6 +41,11 @@ LaneT extract_lane(const LeafT & leaf, std::size_t lane) noexcept
 
 /// @brief zero `out` is the caller's job; this writes one lane and leaves
 ///        every other lane untouched.
+/// @tparam LaneT lane type
+/// @tparam LeafT leaf type
+/// @param leaf the leaf value
+/// @param lane the lane index or lane value
+/// @param value the value to convert or store
 template <typename LaneT, typename LeafT>
 HEDLEY_ALWAYS_INLINE
 HEDLEY_NO_THROW