v1.1.2

fix: gate Unix-only madvise readahead behind cfg(unix)

memmap2::Advice / Mmap::advise are Unix-only, breaking the Windows build.
Make the readahead a no-op on non-Unix targets. Bump to 1.1.2.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>