Region fade between rebuilds + compute off switch

The region bitmap rebuilds only every REGION_EVERY-th frame while the
live sim runs, so during drags the tints stepped at 1/3 the frame
rate. Between rebuilds draw() now linearly blends the previous final
bitmap into the current one ('lighter' + globalAlpha weights = a true
lerp of the premultiplied pixels — no source-over alpha stacking).
Because box blur and the end-alpha scale are linear operators, lerping
the finished bitmaps is equivalent to fading coverage before the blur,
the mathematically exact form of the user's preferred pipeline. Two
tiny GPU blits per frame; splat/blur/readback still only on rebuilds.

New RegionTuning.enabled is a compute switch (not a paint toggle):
off skips the entire build (splat, blur, readback, blit) — the settings
panel exposes it as "Background colors". Cluster labels stay decoupled
and follow clusterLabels. Old persisted files predate the field and
sanitize to enabled: true.

components/graph-view.tsx

@@ -292,11 +292,26 @@ function GraphSettingsControl({
 						    dropdown for the bitmap resolution (it's a discrete
 						    choice, not a free number). Server-persisted like the
 						    sizes; every range end was sweep-validated on the real
-						    graph. */}
+						    graph. The enable switch is a COMPUTE switch: off skips
+						    the whole build (splat/blur/readback/blit), not just
+						    the paint. */}
 						<div className="mt-1.5 flex flex-col gap-1.5 border-t pt-1.5">
 							<span className="text-[10px] font-medium uppercase tracking-wide text-muted-foreground">
 								Regions
 							</span>
+							<label className="flex cursor-pointer items-center justify-between text-[11px] text-muted-foreground">
+								<span>Background colors</span>
+								<input
+									type="checkbox"
+									checked={settings.regions.enabled}
+									onChange={(e) =>
+										changeSettings((cur) => ({
+											...cur,
+											regions: { ...cur.regions, enabled: e.target.checked },
+										}))}
+										className="accent-primary"
+								/>
+							</label>
 							<SettingSlider
 								label="Dilation"
 								min={0}