reachscan: a committed-prefix future-field measurement instrument

Reusable instrument for measuring the future field of a committed reasoning prefix: target reachability, basin structure, and foreclosure under commitment. Ships a token-LLM source, reference projections, projection-pack support, prediction-contract evaluation, and a worked floor-sum example. Sibling to the oracle-backed random 3-SAT constructive-accessibility instrument. v0.3.5 is a cleaned, citable software release intended to support the Core Research Article publication line.