Move `mod tokio_util` to runtime

Editor assessment

Move mod tokio_util to runtime states the relocation, and the body supplies the reason - it avoids a bunch of duplicated code - which, though brief, is genuine motivation rather than restatement and tells the reader the consolidation intent. The refactor type fits, the missing scope is tolerable since the module is named, and hygiene is clean with no footers required. Clear and correct, a publishable refactor example. The duplication rationale, brief as it is, gives the move a purpose and separates it from arbitrary reorganization.