README: tighten Quick Start 3 phrasing

v2.3: croissant url -> HF dataset page; README adds 'Representative sample for review' section per NeurIPS E&D > 4 GB requirement

v2.1: README — fix splits/probe paths, add L2-norm note, add reproduction quick-start with balanced-subsample pattern

v2: refresh README + croissant.json for expanded artifact set

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initial commit