Brummel
dfc01bf837
Four tasks from spec 0086: (1) named non-negativity diagnostic via a parse_nonneg_rate helper + perturbed-test fix; (2) non-R-harness guard (the spec_gap, reject/exit-2); (3) COST_FLAGS_NOTE appended to the usage + --help surfaces; (4) the C10 ledger note + full-suite/clippy gate. Confined to crates/aura-cli; the eight stage1-r cost goldens stay byte-green. refs #153