fix: guard Unix-only grp import behind sys.platform check for Windows compatibility

[Copilot]

init_host_environment.py unconditionally imports Python's built-in grp module, which is Unix-only. On Windows this raises ModuleNotFoundError at import time, blocking the entire CLI before argument parsing — koru --help, --version, and --doctor all fail immediately.

Changes

• src/koru/init_host_environment.py — replace top-level import grp with a platform-conditional import:

  if sys.platform != "win32":
      import grp as _grp
  else:
      _grp = None  # type: ignore[assignment]

  Update _uinput_snapshot() to guard the grp.getgrgid() call with _grp is not None, falling back to the raw numeric GID string. On Windows /dev/uinput is absent anyway so the guard is never reached in practice.

• tests/test_init.py — two new tests:

  • test_grp_sentinel_matches_platform: verifies _grp is None on Windows and the real grp module elsewhere.
  • test_uinput_snapshot_absent_device: verifies _uinput_snapshot() never raises and always returns a dict containing "present", regardless of platform.

[MatthiasLew]

Verified on native Windows 11 with Python 3.13.14 and PowerShell 5.1. The original blocking grp import issue is resolved: koru --help, koru --version, and koru --doctor --project . now start successfully. All 29 tests in tests/test_init.py pass. The remaining full-suite Windows failures are unrelated platform-specific issues and should be handled separately.