Add chrono helpers for Windows time

[antmor]

Summary

• add checked chrono conversion helpers for finite Win32 millisecond timeouts and telemetry-friendly floating milliseconds
• add chrono overloads for WIL event waits
• when C++/WinRT is available, add checked relative and absolute threadpool timer scheduling using winrt::clock and winrt::file_time
• preserve C++17, no-exception, Win7/downlevel, and WIL_NO_CHRONO containment

Design

This is a focused replacement proposal informed by the Windows clock work and review feedback in #89. It does not introduce another Windows clock or FILETIME wrapper: C++/WinRT integrations use the existing winrt::clock and winrt::file_time types directly.

The generic duration helpers are available without C++/WinRT:

• try_to_dword_ms, to_dword_ms_failfast, and exception-enabled to_dword_ms convert finite timeouts, round positive fractional milliseconds upward, reserve INFINITE, and reject negative or overflowing values.
• to_float_ms converts integral durations to fractional milliseconds for measurement and telemetry.
• WIL event waits accept supported chrono durations and delegate to the checked millisecond conversion.

To keep conversion behavior simple and predictable, input durations must use a signed integral representation and a reduced period conversion that requires multiplication or division, but not both. Floating-point, unsigned, and unusual mixed-ratio inputs are rejected at compile time; callers can explicitly duration_cast when intentional.

When <winrt/base.h> is available, re-including <wil/chrono.h> enables the Windows-time surface:

• relative durations are encoded through winrt::file_time for SetThreadpoolTimer;
• absolute deadlines accept winrt::clock::time_point or std::chrono::system_clock::time_point and convert through winrt::clock;
• timer period and window values use the same checked millisecond conversion.

No wil::clock, WIL FILETIME wrapper, SYSTEMTIME wrapper family, custom formatter, or general arbitrary-ratio multiply/divide implementation is added. Callers can use the existing Win32 FILETIME/SYSTEMTIME APIs directly and format with std::format after winrt::clock::to_sys(value).

Validation

The focused chrono tests build and run in these configurations:

Target	Configuration
witest	Normal C++17
witest.noexcept	Exceptions disabled, STL enabled
witest.win7	Win7/downlevel target
witest.cpplatest	Latest C++ and C++/WinRT

Coverage includes timeout ceiling and overflow behavior, telemetry conversion, compile-time rejection of unsupported duration representations and ratios, chrono event waits, relative FILETIME encoding through winrt::file_time, live relative and absolute threadpool timer callbacks, winrt::clock/system_clock conversion, and C++20 std::format usage.

Latest focused result: 96 assertions across 14 test-case executions (18 assertions / 3 cases in each C++17 target and 42 assertions / 5 cases in the latest C++/C++WinRT target).