Function save_probe_cache 

pub(crate) fn save_probe_cache(
    cache_dir: &Path,
    model_variant: &str,
    max_seq: usize,
    a: f64,
    b: f64,
)

Saves fitted probe coefficients to {cache_dir}/probe-coefficients.json via an atomic temp-file + rename.

Errors are logged and silently ignored — a cache write failure must never abort the server.