Cleanup: check for integer overflows on user inputs

Thanks for mentioning the arithmetic thing! Indeed we have some number of arith issues that have slipped in over time and we should do a cleanup pass at some point. For now, the semantics seem reasonable enough (i.e., we can just hard set them to checked/wrapping via a compile time flag depending on what we need) but a cleanup would be good. The harder part is longer-term enforcement which I don't have a good solution for (there are some very good reasons to use normal arith, and we can't just ban +/-/...). Also, briefly: when looking for places to cleanup, we should also look for places where saturating_* is the right move (not just checked_* or wrapping_*). For example, one of the changes I'm working on rn has such a natural use case (let available = data.len().saturating_sub(offset);).

Originally posted by @jaybosamiya-ms in https://github.com/microsoft/litebox/issues/668#issuecomment-3923925897

No files or tests are named. Start by locating arithmetic on user inputs across the Rust codebase, then review each site for checked, wrapping, or saturating behavior. Done means the cleanup scope is addressed and the selected arithmetic semantics are covered by the existing relevant tests.