Add aare::ssize_t alias for MSVC

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.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/decode.test.cpp

@@ -10,6 +10,7 @@ using Catch::Matchers::WithinAbs;
 #include <vector>
 
 using aare::BitOffset;
+using aare::ssize_t;
 
 TEST_CASE("test_adc_sar_05_decode64to16") {
     uint64_t input = 0;