Split the Windows-DLL slice out of T-25 (cross-OS libtree-sitter
build). The FFI loader already resolves tree-sitter.dll on Windows,
but no Windows binary is vendored — only the Linux .so — so
tree-sitter highlighting silently falls back to plain text on
Windows. T-440 is the narrow task to build and vendor that DLL now;
CI reproducibility and the macOS .dylib stay on the T-25 umbrella.
Co-Authored-By: Claude <noreply@anthropic.com>