v0.15.2 — the slider actually drives the model now

The model still only updated when the slider stopped. 0.15.1 fixed one half of this and it wasn't enough, because both halves were wrong.

The scheduler was a debounce: every knob change assigned _rebuildAt = now + delay, pushing the deadline forward with the input. A deadline that moves every time you move the slider never arrives until you stop — which is the symptom exactly.

It now keeps the earliest pending deadline, making the same 250 ms a cap on how often a metered rebuild fires rather than a wait for silence. The local path is unaffected; its delay was already zero.