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.