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.

src/core/render-settings.ts

@@ -100,6 +100,11 @@ export interface RegionTuning {
 	 *  of any count never stack (DECIDED 2026-09-14 — per-stroke alpha
 	 *  accumulated through source-over and double-counted orphans). */
 	edgeAlpha: number;
+	/** THE off switch (user 2026-09-15): false = the renderer never runs
+	 *  the region build — no splat, no blur, no readback, no bitmap
+	 *  allocation, no blit. Compute skipped, not just paint hidden.
+	 *  Cluster labels are decoupled and follow `clusterLabels`. */
+	enabled: boolean;
 }
 
 export const REGION_BITMAP_OPTIONS = [128, 192, 256, 384, 512] as const;
@@ -110,6 +115,7 @@ export const DEFAULT_REGION_TUNING: RegionTuning = {
 	blur: 6,
 	bitmap: 384,
 	edgeAlpha: 0.35,
+	enabled: true,
 };
 
 /** Nearest member of a fixed option set (dropdowns have no free values). */
@@ -129,6 +135,9 @@ export function sanitizeRegionTuning(input: Partial<RegionTuning> | null | undef
 		blur: snapToOption(num(input?.blur, d.blur), REGION_BLUR_OPTIONS),
 		bitmap: snapToOption(bitmap, REGION_BITMAP_OPTIONS),
 		edgeAlpha: Math.min(Math.max(num(input?.edgeAlpha, d.edgeAlpha), REGION_EDGE_ALPHA_RANGE.min), 1),
+		// Old persisted files predate the switch: missing → on (the look
+		// users tuned); only an explicit false turns regions off.
+		enabled: typeof input?.enabled === "boolean" ? input.enabled : true,
 	};
 }