Function parameter_slots 

pub fn parameter_slots(bytecode: &Bytecode, chunk_idx: usize) -> u16

Parameter / this slot count for a chunk: the translator’s starting stack height (RegChunk::param_slots), so let locals land on their own registers.

The register VM and the disassembler must agree on this, so it lives next to the translator. arity on a RegChunk stays 0 for chunks reached through CallDirect (a deep call has no FunctionObj to read), which is why the count is recovered from the constant pool instead.