Function diff_specs 

pub fn diff_specs(
    old: &ContractSpec,
    new: &ContractSpec,
    old_summary: WasmSummary,
    new_summary: WasmSummary,
) -> Result<SpecDiff, WasmError>

Diff two already-parsed ContractSpec values.

old_summary / new_summary carry WASM identity (hash/size) for context in reports; pass WasmSummary::default() when not available.