util/SpanCast: add const support to FromBytesFloor()

src/util/SpanCast.hxx

@@ -33,13 +33,16 @@
 #include <span>
 #include <string_view>
 
+template<typename From, typename To>
+using CopyConst = std::conditional_t<std::is_const_v<From>, const To, To>;
+
 /**
  * Cast a std::span<std::byte> to a std::span<T>, rounding down to the
  * next multiple of T's size.
  */
 template<typename T>
 constexpr std::span<T>
-FromBytesFloor(std::span<std::byte> other) noexcept
+FromBytesFloor(std::span<CopyConst<T, std::byte>> other) noexcept
 {
 	static_assert(sizeof(T) > 0, "Empty base type");