Five partial_cmp().expect("no NaN") sorts are latent panics; total_cmp is free

[vyncint]

Five sorts use partial_cmp(..).expect("no NaN") on f64 costs, medians and speedups: cli/main.rs:629, main.rs:777, tui/app.rs:305, report/build.rs:136, build.rs:172. Today's model produces finite costs or INFINITY (which orders), so the expect does not fire — but a NaN from a future model term or a malformed results JSON turns a report into a panic. f64::total_cmp is a total order, free, and makes the question moot.

Done when

• No partial_cmp(..).expect remains; total_cmp in its place, with NaN documented as sorting last.
• A test feeds a NaN cost through the ranking and the report and gets a result, not a panic.