ci: add Cygwin 1.2.x build coverage

[somethingwithproof]

Summary

Adds a separate Cygwin/Windows support PR for the 1.2.x branch.

• add a Windows-hosted GitHub Actions workflow that installs 64-bit Cygwin dependencies, runs bootstrap, configures against /usr, and builds with verbose output
• make configure.ac explicitly support only x86_64 Cygwin and fail clearly for unsupported 32-bit Cygwin hosts
• clarify documentation that Spine 1.2.x supports Cygwin on Windows, not native Windows builds
• preserve existing CYGWIN environment options while adding nodosfilewarning when missing
• store the selected Cygwin shell path explicitly instead of using an integer location flag
• use select(fd + 1, ...) in poller/ping paths instead of FD_SETSIZE

Validation

• git diff --check
• workflow YAML parse with Ruby YAML.load_file
• local C syntax check with installed MySQL/MariaDB, Net-SNMP, OpenSSL headers and -DHAVE_LIBPTHREAD=1

The actual Cygwin build is expected to run in GitHub Actions after this PR is opened.

[Copilot]

Pull request overview

This PR adds Cygwin (Windows) build coverage for the 1.2.x branch by introducing a dedicated GitHub Actions workflow and tightening/clarifying Cygwin support across the build system, docs, and runtime behavior.

Changes:

• Add a Windows-hosted GitHub Actions workflow to build Spine under 64-bit Cygwin.
• Restrict configure.ac to explicitly support only x86_64 Cygwin and fail fast on unsupported Cygwin hosts.
• Improve Cygwin runtime behavior by preserving existing CYGWIN env options while adding nodosfilewarning, and by storing the selected shell path explicitly.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
spine.h	Updates Cygwin stderr comment and replaces the Cygwin shell location flag with an explicit stored shell path.
spine.c	Preserves CYGWIN env options while appending nodosfilewarning, and records the resolved Cygwin shell path.
README.md	Clarifies that Windows support is via 64-bit Cygwin (not native Windows) and updates dependency/docs wording.
poller.c	Uses select(cmd_fd + 1, ...) instead of FD_SETSIZE when waiting on the script pipe FD.
ping.c	Uses select(sock + 1, ...) for ICMP/UDP select calls instead of FD_SETSIZE.
nft_popen.c	Uses the configured Cygwin shell path for execve() rather than a numeric location flag.
INSTALL	Updates Windows/Cygwin installation and support documentation to match 1.2.x expectations.
configure.ac	Adds explicit x86_64 Cygwin handling and a clear error for unsupported Cygwin hosts.
.github/workflows/cygwin.yml	Introduces a new CI workflow to build on windows-latest with 64-bit Cygwin dependencies.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[somethingwithproof]

Folded into #544. The Cygwin job exists to validate the fork()/FD_CLOEXEC changes in that PR, so they belong together.

The out-of-scope portability work that was riding along here (the configure.ac 32-bit rejection, the cygwin_sh_path struct change, and the select(fd + 1) edits) is split out to split/cygwin-portability-1.2.x and will come as its own PR.