Run most of test_future on Linux

[Copilot]

All of test_future was excluded on non-Windows via a blanket #![cfg(windows)] even though most tests only exercise the stock windows-future async implementations, which are pure Rust apart from one internal Waiter and a thread-pool dispatch that already had non-Windows paths.

windows-future

• New cfg(all(not(windows), feature = "std")) Waiter/WaiterSignaler backed by Arc<(Mutex<bool>, Condvar)> so IAsyncAction::join() works off-Windows.
• Gated bindings_impl (raw link! decls for CreateEventW/SetEvent/WaitForSingleObject/CloseHandle and HANDLE/BOOL/...) behind #[cfg(windows)].
• Crate's std feature now also enables windows-threading/std so the std::thread::spawn fallback for submit is actually pulled in.

test_future

• Replaced use windows::core::* / use windows::Win32::Foundation::* with use windows_core::* plus inline const HRESULT definitions for E_FAIL, E_ILLEGAL_DELEGATE_ASSIGNMENT, E_ILLEGAL_METHOD_CALL, E_PROTOCOL_EXTENSIONS_NOT_SUPPORTED (matches the existing pattern in test_collections). The E_FAIL constant in when.rs is gated #[cfg(windows)] to match its only consumer (the err test).
• Dropped #![cfg(windows)] from agile, async_info, completed_finished, completed_get, completed_once, completed_started, dropped, progress, started, when, and the empty src/lib.rs.
• Kept Windows-only:
  • futures.rs — uses Win32 ThreadPool and WinRT Storage::Streams.
  • error.rs and when::err — assert on Error::message(), which only round-trips message text on Windows (via IErrorInfo/OriginateError).
• Added windows-core to dev-dependencies.

Result on Linux

10 of 12 test binaries now run; 29 cross-platform tests pass. Windows behavior is unchanged — every new branch is gated by #[cfg(windows)] / #[cfg(all(not(windows), feature = "std"))].

[Kenny Kerr (kennykerr)]

Copilot check build

[Copilot]

Copilot check build

Two CI failures, both fixed in 93e7b72:

• fmt: crates/tests/libs/future/src/lib.rs was empty (needed a trailing newline).
• test (stable linux): E_FAIL const in when.rs was unused on Linux because the only consumer (err) is #[cfg(windows)]. Gated the const the same way.