Reframe bench/README.md from maintainer-facing internals to a reader-facing
overview: what the benchmark measures, how it works at a high level, and what
it found. Add the completed 4-arm results (240 samples, 2026-07-17).
Remove the per-file layout, the vocabulary glossary, the packaging note, the
spec/ADR pointer, and the run/report/test command sections — that detail lives
in the code and the spec, and it buried the point. The result is a one-screen
doc: pitch and headline finding, a one-paragraph methodology, and the results
table sorted by cost-equivalent tokens.
