ref(hwi): move descriptor utils into descriptor utils

ref(hwi): move descriptor utils into descriptor utils

- Move the single-sig script-type logic and multipath detection
  out of the HWI handler and into utils/descriptors.

- Add SingleSigScriptType (ValueEnum) with purpose/as_str/descriptor
  and from_derivation_path.
- Add is_multipath_descriptor, a wrapper over the existing
  descriptor_path_count primitive.
- Drop the contains_private_key helper; the final DescriptorPublicKey
  parse already rejects private keys.
- Add BitBox simulator endpoint as a const.