Changed koffi import from top-level to dynamic require in
enableWindowsVTInput() and added --external koffi to bun build.
This prevents embedding all 18 platform .node files (~74MB) into
every compiled binary. For Windows builds, only the win32_x64
koffi.node is shipped alongside the binary.
Binary size reduction: darwin-arm64 142MB -> 67MB, archive 43MB -> 28MB.
8386a807ff