#153 (friction surfaced by the milestone-#148 close fieldtest). Three cost-flag
fixes on `aura run`, all on the pure parse_run_args grammar:
- Units discoverable: a COST_FLAGS_NOTE appended to the run usage + --help
(price units, charged in R as cost/|entry-stop|, per-ENGINE-cycle carry,
>= 0, stage1-r only).
- A negative rate names the cause + flag ("<flag> must be non-negative, got <v>")
via a factored parse_nonneg_rate helper, not a bare usage dump.
- A cost flag on a non-R harness (sma/macd) is refused (exit 2, "cost flags
require an R-evaluator harness (stage1-r)") instead of silently ignored — the
named spec_gap, decided reject/fail-loud (refuse-don't-guess, recorded on #153),
pinned by a new C10 ledger note.
Internal RunArgs/RunData/HarnessKind gain #[derive(Debug)] (behaviour-neutral) so
the pure parse_run_args unit tests can use expect_err.
Verified independently: cargo test -p aura-cli green, the eight stage1-r cost
goldens byte-unchanged; clippy -p aura-cli --all-targets -D warnings clean. Only
the grammar/usage strings and the previously-unguarded sma-cost path changed;
the cost-model graph and R math are untouched.
closes#153
Brummel
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