A Reproducible Extension Beyond 2^71 and Exact First-Descent Statistics for the Collatz Map

This preprint gives a reproducible finite extension of the published Collatz verification boundary. Using Barina's open-source verifier with two residue-sieve configurations, every integer in [2^71, 2^71 + 2^40) is certified to descend below its starting value; combined with the published base computation, this verifies convergence for every n < 2^71 + 2^40. A separate direct computation enumerates the exact first-descent distribution for all 2^34 integers in [2^71, 2^71 + 2^34). The observed survival tail motivates a conjectural large-deviation law and an associated extreme-value heuristic. The probabilistic discussion is heuristic and does not prove the unrestricted Collatz conjecture. This revised version adds a direct comparison with David Barina's distributed verification project, distinguishing its full-prefix computation and infrastructure from the present adjacent work-unit extension and clarifying that the two local runs share the same upstream source logic. The accompanying source archive contains the LaTeX manuscript, exact JSON data, block certificates, C++ and Python code, and SHA-256 checksums.