Decomposes the boss-signed spec into four tasks: (1) the `Source` trait +
`VecSource` adapter in aura-engine; (2) the atomic `Harness::run` re-type to
`Vec<Box<dyn Source>>` with all 53 call sites threaded behaviour-preserving;
(3) the streaming `M1FieldSource` over a data-server window; (4) a gated
streaming e2e + the measured residency predicate.
refs #71
Brummel
9337a8585d