Function script_sig_args_expected 

fn script_sig_args_expected(kind: &ScriptKind) -> Option<usize>

Returns the expected number of scriptSig arguments for a given script kind.

TODO: Consider upstreaming to zcash_script crate alongside ScriptKind::req_sigs().

Mirrors zcashd’s ScriptSigArgsExpected(): https://github.com/zcash/zcash/blob/v6.11.0/src/script/standard.cpp#L135

Returns None for non-standard types (TX_NONSTANDARD, TX_NULL_DATA).