Full Windows portability: proper implementations, not stubs

- handle_processes: GetProcessMemoryInfo + GetProcessTimes (Windows),
  getrusage (POSIX) — real process metrics on both platforms
- handle_process_kill: TerminateProcess via OpenProcess (Windows),
  kill(SIGTERM) (POSIX)
- index_thread_fn: CreateProcess + WaitForSingleObject (Windows),
  fork+exec+waitpid (POSIX)
- cbm_clock_gettime: QueryPerformanceCounter (Windows) — replaces
  all bare clock_gettime calls across 6 files
- cbm_nanosleep, cbm_strcasestr, cbm_mkdir: compat shims
- CBM_TLS: _Thread_local (C11 standard, works on all compilers)
- Replace bare strndup with cbm_strndup, guard POSIX headers
- Add mingw-w64 cross-compile to Docker test infrastructure