FLX-4103: fidelity: stochastic declared with no sweep executor available

Family: governance/sovereignty (the 4000 band)

Simulation.spec.fidelity: stochastic (schema-declared, escalating past deterministic) requires a sweep executor that runs one Simulation per seed in ("sweep", run_index) (KERNEL-SPEC section 3) and aggregates the results. This build ships the seed-tree primitives (sweep_effective_seed/sweep_path) but no executor yet -- run one Simulation.spec.seed at a time (fidelity: deterministic) until a sweep executor is available.

Look it up offline

$ fluxsim explain FLX-4103

Every code is listed in the error index.