Region alpha applies once after the blur: no more overlap stacking

Per-stroke alpha baked into every stroke/disc accumulated through
source-over compositing: one shape 0.35, an orphan disc over anything
1-0.65^2 ~ 0.58, hubs toward 1.0 — a lone node's glow read as a
darker double-counted spot on top of other regions. The pipeline is
now dilate at full opacity -> blur -> scale the whole field by
edgeAlpha once at the end. Painted at 1, the alpha channel is a pure
coverage indicator (overlaps of any count contribute exactly 1.0),
the blur smooths it into a coverage falloff, and the final scaling
gives a constant interior alpha everywhere covered, varying only
across the blur edges. The blur-off path also runs the readback now
(radius 0 = copy) so the end-alpha applies uniformly without blur.

Interaction change (user-flagged, accepted for now): different
clusters' overlapping halos no longer blend hues proportionally —
the later-painted cluster covers the earlier, ownership boundaries
softened only by the blur reach. Verified: old build spread 36k of
55k tinted pixels above alpha 100 (stacking); new build concentrates
at 70-90 (0.35*255=89 interior with falloff), and an orphan disc now
reads no brighter than the skeleton it sits on.

src/core/render-settings.ts

@@ -95,7 +95,10 @@ export interface RegionTuning {
 	/** Max bitmap dimension (cell = worldSpan / this). Fixed set:
 	 *  see REGION_BITMAP_OPTIONS (dropdown in the panel). */
 	bitmap: number;
-	/** Per-stroke alpha; overlapping strokes stack toward the core. */
+	/** Region alpha, applied ONCE after the blur (dilate at full
+	 *  opacity → blur → scale). Constant interior, soft edges; overlaps
+	 *  of any count never stack (DECIDED 2026-09-14 — per-stroke alpha
+	 *  accumulated through source-over and double-counted orphans). */
 	edgeAlpha: number;
 }