Add bench/aggregate.ts: the pure aggregator seam that rolls the
accumulated sample store into a readable comparison. aggregate produces
a headline table (one row per arm, cost-equivalent tokens as the
headline plus raw tokens, turns, duration, success rate, coverage, and
imputed cost as a de-emphasized secondary column), per-tier and
per-token-component breakdowns, and the separate bonus table; renderReport
renders it as stable text. Cost-equivalent tokens are weighted at render
time from the four retained components per ADR 0014, and incomplete
coverage is annotated rather than hidden. Unit-tested against synthetic
append-only sample stores.
