fix(chunked_fuse): lazy fallback to st_ctime for macOS crtime field

[jackd]

Full disclosure: I'm not a rust guy, nor am I a mac person. An agent proposed this to resolve a build failure I was having, other agents agreed, and having made the change the build now succeeds. Below is a summary generated by one such agent.

Summary

On macOS, fuse3's FileAttr struct requires two extra fields not present on Linux: crtime (file creation time) and flags. This PR adds both.

For crtime, the implementation prefers st_birthtime from the Python stat dict when available (e.g. on real macOS filesystems), and falls back to st_ctime when the key is absent (e.g. when the remote filesystem or Python stat implementation doesn't populate it). This avoids panics/errors when mounting a FUSE filesystem on macOS against a remote that doesn't expose birthtime.

flags is set to 0 as there are no relevant BSD file flags to propagate.

Both fields are gated with #[cfg(target_os = "macos")] so Linux builds are unaffected.

Changes

chunked_fuse.rs: In extract_attr, conditionally populate crtime (with lazy st_ctime fallback) and flags on macOS.

Testing

Compile-tested on macOS. The fallback path handles filesystems that don't expose st_birthtime without returning an error.

Note: USE_TENSOR_ENGINE=0 uv sync resulted in an updated uv.lock package entry for name = "py-spy". I assume this is unrelated to these changes and is down to an outdated uv.lock file in the repo.

[jackd]

@cpuhrsch possibly most relevant?