Function probe_ready_toolchain 

pub async fn probe_ready_toolchain(
    pkg: &str,
    _platform: ToolPlatform,
    cache_dir: &Path,
) -> Option<ToolchainHandle>

Non-blocking, filesystem-only probe for an already-provisioned keg.

Returns Some(handle) iff a .ready-stamped keg for (pkg, platform, cache_dir) exists on disk; otherwise None. Does no network I/O, no build, and no mutation — it reconstructs the ToolchainHandle from the keg’s manifest exactly as the fast path of ensure_toolchain does.

It exists so a caller that provisions in the background (and therefore can’t await the cold install on its hot path) can still inject a keg already on disk — e.g. from a previous daemon process. Returns None for any non-macOS platform or when no ready keg is present.