Function parse_draft_conf_tau 

pub fn parse_draft_conf_tau(value: Option<&str>) -> f32

The rule itself, pure over the raw value.

Split from the reader so a test can exercise the CLAMP without mutating the process environment — set_var is global and cargo test runs this binary’s tests in parallel, so an env-mutating test races every other one. The clamp is the load-bearing part: an unclamped 5.0 would put the floor above any achievable confidence and discard EVERY draft, turning speculation off with nothing logged.