Add aare::ssize_t alias for MSVC (#398)

- MSVC has no ssize_t, which made every translation unit fail on
Windows.
- Define aare::ssize_t in defs.hpp: the platform ssize_t on POSIX (so
nothing changes there) and std::ptrdiff_t on MSVC.
- Code inside the namespace picks it up unchanged; the few files that
use ssize_t outside the namespace get a using-declaration. FilePtr.hpp
now includes defs.hpp since it had no aare include.

Closing #393

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

src/NDView.test.cpp

@@ -11,6 +11,7 @@
 using aare::NDView;
 using aare::num_elements;
 using aare::Shape;
+using aare::ssize_t;
 
 static_assert(std::is_convertible_v<NDView<int, 2>, NDView<const int, 2>>);
 static_assert(!std::is_convertible_v<NDView<const int, 2>, NDView<int, 2>>);