Function lookup_int64 

pub fn lookup_int64<A: Accessor>(
    registry: &Registry,
    accessor: &A,
    concept: Concept,
) -> Option<i64>

Look up a concept as an i64.

Mirrors upstream LookupInt64:

• Int64 tags match only integer-typed attributes.
• Float64 tags accept float-typed attributes whose value is losslessly representable as an i64.
• String tags accept string-typed attributes, parsing via i64::from_str first and then a float-parse fallback for integer-valued floats.