[Youw]

Move the Cygwin build from AppVeyor to a new windows-cygwin job in GitHub Actions.

The Cygwin job was the slowest part of every AppVeyor build: 4-6 minutes, about 3 of them spent upgrading the preinstalled Cygwin, while each MSBuild job takes under a minute. AppVeyor runs one build at a time, so builds also queue behind each other (up to 10 minutes for recent PRs), and AppVeyor was the last check to report on most pushes and PRs.

Coverage compared with the AppVeyor job:

• Same: current Cygwin x86_64 with GCC, ./bootstrap && ./configure && make, building the Windows backend (libhidapi.la, shared and static) and hidtest.
• Added: compiles with the same -Wall -Wextra -pedantic -Werror -Wformat-signedness as the other GCC jobs. Cygwin is the only Windows toolchain in CI with a 64-bit long, so its warnings can differ from MinGW's.
• Added: make install, then checks the installed DLL, import library, header and pkg-config files.
• Added: runs hidtest as a runtime smoke test under Cygwin. The hosted runner exposes a virtual HID mouse, so this enumerates a real device and rebuilds its report descriptor from preparsed data. The device hidtest tries to open isn't there, so exit code 1 ("unable to open device") is the expected result.
• The build runs with a Cygwin-only PATH, so no toolchain from the runner image can leak into it.

AppVeyor keeps the two Visual Studio 2015 MSBuild jobs, the oldest MSVC in CI. With only MSBuild left, the for: matrix in .appveyor.yml is no longer needed.

Timing, measured on this branch and on the last five master builds:

	Before	After
AppVeyor build	5.2-7.8 min (Cygwin job 4.2-6.0 min)	1.2-1.3 min (two MSBuild jobs)
GitHub Actions builds.yml	2.3 min	2.2-2.3 min (windows-cygwin itself: 1.9-2.3 min)

So all checks report about 2.5 minutes after a push instead of 5-8. Each AppVeyor build also holds AppVeyor's single build slot for about a minute instead of about six, which shortens the queue for builds waiting behind it.

Validation:

• Branch run: all builds.yml jobs pass; the log shows the warning flags on both hid.c compiles and on the hidtest link.
• AppVeyor passed on all three branch commits with the trimmed configuration.

After merging, windows-cygwin should be added to the required status checks for master. Until then a Cygwin failure won't block a merge; today it does, through continuous-integration/appveyor/pr.

Assisted-by: claude-code:claude-opus-5-5

Generated by Claude Code (Claude Opus 5.5).