Range sizing × format

Probes N URLs per format through prepare(url, { strategy: 'range', rangeBytesByFormat }), varying the bytes requested per format. Each probe is cache-busted. Reports per-format time distribution, success count, failure categories (CORS, network, probe-yielded-no-dims), and the effect of the library's re-range fallback. Defaults come from DEFAULT_RANGE_BYTES_BY_FORMAT, which was itself derived from the byte-threshold corpus sweep.