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/ClusterVector.test.cpp

@@ -12,6 +12,7 @@
 
 using aare::Cluster;
 using aare::ClusterVector;
+using aare::ssize_t;
 using C1 = Cluster<int32_t, 2, 2>;
 
 TEST_CASE("A newly created ClusterVector is empty") {