Two tasks: (1) aura-engine SplitMix64 PRNG + pub SyntheticSpec::source(seed) ->
impl Source beside VecSource, with producer RED tests; (2) aura-cli
sim_optimal_manifest gains a seed param (four callers pass 0), plus a test-only
run_sample_seeded + SeededTrace and three e2e RED tests pinning the #66
acceptance. RED-first throughout.
refs #66
