Function offset_map 

pub fn offset_map(source: &str) -> OffsetMap

Build the exact OffsetMap for source.

Re-derives the sanitize transform’s offset shifts pass by pass. Pure — allocates only the small per-pass anchor tables (empty for a source that needs no sanitation).

The PUA-sentinel neutralization (the final sanitize pass) is byte-length-preserving, so it contributes no shift and is not modelled.