Add JAX tracing to optional CI matrix (U-006)

[anulum]

Context

src/scpn_control/control/jax_traceable_runtime.py provides JAX-traced versions of key functions but JAX is not in the CI dependency matrix, so tracing correctness is untested.

Acceptance criteria

• Add optional CI job that installs jax[cpu] and runs JAX-specific tests
• Verify traced functions produce outputs matching NumPy reference within tolerance

Tracked in UNDERDEVELOPED_REGISTER.md as U-006.