Add cNMF GPU consensus test coverage

[JerryIshihara]

Summary

Adds test coverage for the GPU consensus/refit path in cNMF.

Changes

• Validates GPU CLI options are accepted for both factorize and consensus
• Keeps GPU options rejected for non-engine commands like prepare, combine,
  and k_selection_plot
• Tests fixed-H GPU kernel behavior for consensus refit:
  • update_H=False preserves supplied spectra H
  • usages W are updated with fixed-H MU
  • invalid fixed H inputs raise clear errors
  • fixed-H compile path uses _mu_step_fixed_h
  • fixed-H fit loop respects early stop and max-iter bounds
• Adds cNMF integration tests for:
  • refit_usage() GPU wiring
  • refit_spectra() transposed GPU wiring
  • tiny CPU-backed GPU consensus smoke outputs
  • default CPU consensus path not leaking engine/gpu kwargs