[PERF]: cuCascade byte range prefetch backend for hybrid scan

Experimental byte range prefetch backend using cuCascade's reactor-based IO engine. Uses fadvise hints to trigger downloads into cuCascade's internal prefetch cache.

PR: #23450

Start with PR #23450, since the issue identifies it as the implementation location; no source files, tests, or entry points are named here. Review how the proposed reactor-based IO engine uses fadvise hints and cuCascade's internal prefetch cache, then confirm that the hybrid scan backend provides the described byte-range prefetch behavior and its validation.

Written by the indexing model from the issue text.