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>

benchmarks/ndarray_benchmark.cpp

@@ -3,6 +3,7 @@
 #include <benchmark/benchmark.h>
 
 using aare::NDArray;
+using aare::ssize_t;
 
 constexpr ssize_t size = 1024;
 class TwoArrays : public benchmark::Fixture {