Test hybrid multi-head backend parity

[darthjaja6]

Summary

• extend the CUDA multi-head parity regression from pure cuEquivariance to the hybrid cuEquivariance/OpenEquivariance conversion
• exercise both DFT and MP2 heads
• compare energy, forces, and stress against the e3nn reference

Context

Issue #1298 reports large and non-deterministic MACE-MH-1 disagreements with both CuEq and hybrid acceleration. The historical multi-head readout leakage is already fixed, and current tests cover pure CuEq, but the reported hybrid path had no equivalent regression coverage.

Validation

• MACE_REQUIRE_CAPS=gpu,cueq,oeq pytest tests/backends -q — 52 passed
• Black, isort, pylint, and git diff --check