Phase 3 (L17). The midi/osc/vcv transports each carried their own copy of the
status plumbing, throttle gate and lastSent management; the inputs layer already
had this factored out as BaseSource, so BaseBackend mirrors its shape rather
than inventing a second convention.
AUDIT CORRECTION: L17 says "all four" backends duplicate lastSent. Only three
do. The uSEQ CV backend has a genuinely different frame-level dead-zone (an
Int32Array over 14 channels plus gate bits) and no per-output lastSent, so it
takes the status half only — forcing it into the shared per-output path would
have needed a special case, and a base class with a per-subclass escape hatch is
worse than three backends sharing one. Also: BaseSource itself has no throttle
or lastSent (it is status + action plumbing only), so the mirror is partial by
nature; the throttle/lastSent half is the duplication the backends actually had.
Three internal representation changes, none observable: the MIDI backend's
lastSent moves Int16Array -> Float32Array (stored values are integers -1..127,
exact in f32, so every dead-zone comparison is bit-identical); the CV backend
now inherits a setContext that also allocates a per-output buffer it never reads
(cold path, no effect); and onStatusChange's unsubscribe returns void rather
than Set.delete's boolean (the declared type was already `() => void` and no
caller used it).
Phase 2's fix to vcv-backend.ts — tracking and dead-zoning the full N-dim input
vector rather than 2 — is preserved.
Gates: typecheck clean, 17/17 unit tests.
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