v0.1.9

Add POSIX compat headers and bundle 7za.exe for MSVC

- compat/include/unistd.h: MIT-licensed MSVC compatibility shim
  (same header previously shipped with vclibs64) so engine source
  can #include <unistd.h> without per-file WIN32 guards.
- vcpkg/build.sh: copy compat/include/ into the x64-windows-msvc
  output, and download 7za.exe (from ip7z/7zip GitHub releases)
  into bin/ so FindSevenZip can locate it at engine build time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>