fix(klu): keep topology indices in numpy so nested sax.circuit traces cleanly

[cdaunt]

When a SAX model composes a sub-circuit via sax.circuit(...) inside its own body and the outer model is itself wrapped in another JAX trace (jax.jit, jax.jacfwd, or any external simulator that traces SAX models for Jacobian assembly), analyze_circuit_klu raised NonConcreteBooleanIndexError at:

CSi = jnp.broadcast_to(Ci[None, :], match_2d.shape)[match_2d]

The boolean fancy-index requires match_2d to be concrete, but it became a tracer because Si/Sj came from sax.scoo(instance) which turned them into tracers under the outer trace.

Index arrays Si / Sj / Ci / Cj are pure netlist topology (which (i, j) positions of each model's S-matrix are non-zero, plus the connection endpoints) — they never depend on traced parameters. Force them to numpy inside the klu backend only — _scoo_with_numpy_indices extracts topology directly from the SDict's keys before SCoo wrapping turns them into tracers, and the rest of analyze_circuit_klu then runs concretely. sax.scoo and the rest of SAX continue to return jnp indices, so other backends are unaffected.

Repro::

def coupler_with_sbends(*, wl=1.55, length=10.0, kappa=0.5):
    sub, _ = sax.circuit(
        netlist={
            "instances": {"s1": "st", "s2": "st", "dc": "cp"},
            "connections": {"s1,o2": "dc,o1", "s2,o2": "dc,o2"},
            "ports": {"o1": "s1,o1", "o2": "s2,o1",
                      "o3": "dc,o3", "o4": "dc,o4"},
        },
        models={"st": straight, "cp": coupler_4port},
    )
    return sub(wl=wl, s1={"length": length / 2}, s2={"length": length / 2},
               dc={"kappa": kappa})

jax.jit(coupler_with_sbends)(wl=1.55)
    # before: NonConcreteBooleanIndexError
jax.jacfwd(lambda w: coupler_with_sbends(wl=w)[("o1","o3")].real)(1.55)
    # before: NonConcreteBooleanIndexError

All 113 existing SAX tests pass.

[cdaunt]

fixes #109