Add Windows support and CI coverage

Summary

• replace POSIX-only filesystem, directory, and child-process helpers with portable Zig APIs
• add Windows time, sleep, argument parsing, path normalization, and shutdown behavior
• cover portable file paths and subprocess exit codes
• add windows-latest to the required test matrix and document supported platforms

Verification

• zig build test -j1 --summary none
• zig build
• zig build examples
• zig fmt --check .
• zig build-lib -femit-docs src/lib.zig
• zig build -Dtarget=x86_64-windows-gnu
• all Windows test and example artifacts cross-compile successfully

Closes #6