Make BoxCallableOnce and BoxRunnableOnce store Send closures by default, and add LocalBoxCallableOnce and LocalBoxRunnableOnce for non-Send local captures. Update Callable/CallableOnce/RunnableOnce conversion APIs, docs, and tests for the new Send/local split.

BREAKING CHANGE: BoxCallableOnce, BoxRunnableOnce, and the default into_box/to_box/into_once/to_once once-task conversions now require Send. Use the LocalBox*Once types or local conversion methods for non-Send captures.