Add Windows support and a Windows CI job #6

Problem

The compatibility layer and process execution rely heavily on POSIX and libc APIs such as fork, execve, opendir, unlink, and signals. The current CI matrix covers Linux and macOS only.

Proposed work

• Replace platform-specific filesystem and process code with current portable Zig I/O APIs where possible.
• Isolate unavoidable platform differences behind a narrow interface.
• Define signal and shutdown behavior on Windows.
• Add a Windows runner to the CI matrix.
• Document supported Zig and Windows versions.

Acceptance criteria

• The library, CLI, tests, and examples build on Windows.
• Discovery can execute test files on Windows.
• Path handling and subprocess exit codes are covered by tests.
• Windows is included in required CI checks.