Optional ReadonlychannelsAn explicit channel list, when the caller already has one (the software mixer's config).
Optional ReadonlyfaderFader taper for every channel; defaults to a fresh NormalizedScale.
Optional ReadonlyfeaturePer-kind feature matrix; defaults to the contract's own featureMatrixOf.
Optional ReadonlylayoutPer-kind counts in surface order — the usual case. Mutually exclusive with channels.
ReadonlymodelHuman model string surfaced on the topology (e.g. "X32", "Roland M-5000").
Optional ReadonlysendsSend routing; defaults to inputs → every audio-bus kind present. See SendSpec.
Optional ReadonlystereoInherent stereo pairs (e.g. the master L/R). Defaults to none.
The spec buildTopology consumes: what channels, what features, what routing.