Turn a benchmark run into a three-arm experiment: alongside the new-skill
and no-skill arms, add a previous-version arm materialized from the
default branch's HEAD, drawn automatically whenever the skill's directory
differs from HEAD and degrading to the two-arm efficacy-only shape
otherwise. Two blind head-to-heads now fall out per trial — efficacy
(new-vs-no-skill) and regression (new-vs-old).
The deterministic core dispatches a pass rule per comparison (efficacy at
wins>=3 and losses<=1, regression at losses<=1 with no wins floor),
gates only Efficacy on the hard assertion, and yields a second
skill-level Regression verdict, green when no case regressed and
not-applicable on a two-arm run. The report gains a second badge, a
headline reading both verdicts together, a previous-version cost row with
a two-ratio footnote, and per-case side-by-side comparisons that
auto-expand on any failure or flagged loss to show the losing-trial
output pair.
The fixture test covers both shapes in one test: a three-arm run
asserting per-arm metrics, both per-case and skill-level verdicts, the
net margins, the three-row cost table and losing-trial evidence, and the
two-arm run retained as the degenerate no-previous-version case.
