Add the longitudinal layer to the benchmark core: a per-skill JSON-lines
history that appends one summary line per run and trims oldest-first at
fifty, two stacked net-margin trend ribbons (Efficacy and Regression, the
latter leaving a gap for two-arm runs) with the current run ringed and a
net/delta/green-count readout, and per-badge fragility chips that flag the
narrowest passing case on each green axis so a barely-green skill cannot
look robust.
History persistence is opt-in via --history and is the core's only side
effect. Without it the core stays a pure transform. Two committed fixtures
(clean and fragile) and new fixture-test sections cover the append-and-trim,
both ribbons including the two-arm gap, and the chips.
