Workstream F core. Pull-based InputSource interface (N axes + discrete
actions), three sources (XYPadSource push-based, WebMidiInputSource with
learn-map CCs/notes, GamepadSource single/double stick), and an InputLayer
that composes active sources into one N-dim vector at the head of the
reactive spine. Arity reduction blends >2 axes down to the fixed 2-input
WASM head; the true multi-WASM reshape is a documented TODO (inputs-spec).
useInputLayer is the React binding (enable/config/status/channel layout).
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