addr::to_usize turns a 64-bit file address or length into a slice index,
failing with FormatError::Overflow where it does not fit usize (32-bit
targets such as wasm32) instead of truncating like an `as usize` cast.
Callers are converted in the following commits.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>