win32: keep a winsock instance alive during tests to fix spurious CI failures

[jcelerier]

Summary

Restores the lost fix (originally e5ea48d) for the spurious Win32 CI test failures caused by WinSock being torn down between operations.

asio refcounts WSAStartup/WSACleanup per object, so during the test run WinSock can be cleaned up between operations, producing intermittent socket failures on the Win32 runners (e.g. https://github.com/ossia/libossia/actions/runs/27264509297/job/80518258354).

The fix keeps a WinSock instance alive for the whole lifetime of the test process via a static owner struct in the shared Catch test header (tests/catch/include_catch.hpp), which every test pulls in. Library code is unchanged.

#if defined(_WIN32)
#include <winsock2.h>
static const struct ossia_test_winsock_owner {
  ossia_test_winsock_owner() { WSADATA d; ::WSAStartup(MAKEWORD(2, 2), &d); }
  ~ossia_test_winsock_owner() { ::WSACleanup(); }
} ossia_test_winsock_owner_instance;
#endif

Test plan

• Win32 test jobs stay green across repeated CI runs (the failures were intermittent).

https://claude.ai/code/session_01XRmDZYxYXvYGHnnAVQ3j9T

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