Add Nengo Loihi emulator CI test (U-002)

[anulum]

Context

src/scpn_control/control/nengo_snn_wrapper.py wraps Nengo for SNN execution but the Nengo Loihi backend is untested in CI because it requires nengo_loihi hardware or emulator.

Acceptance criteria

• Add nengo_loihi emulator test to CI matrix (conditional on nengo_loihi availability)
• Verify SNN wrapper produces identical outputs on CPU vs Loihi emulator backends

Tracked in UNDERDEVELOPED_REGISTER.md as U-002.