feat(effects): add Closing brush as inverse of Opening brush

Allow covering revealed darkness again during darkened scene sessions.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/renderer/shared/effects/SceneDarknessOverlay.tsx

@@ -1,6 +1,11 @@
 import React, { useEffect, useRef } from 'react';
 
-import type { SceneDarknessRevealStroke, SceneDarknessState } from '../../../shared/types';
+import type {
+  SceneDarknessRevealStroke,
+  SceneDarknessState,
+  SceneDarknessStrokeMode,
+} from '../../../shared/types';
+import { normalizeSceneDarknessStrokeMode } from '../../../shared/types/sceneDarkness';
 
 export type SceneDarknessOverlayProps = {
   state: SceneDarknessState | null;
@@ -10,14 +15,18 @@ export type SceneDarknessOverlayProps = {
   style?: React.CSSProperties;
 };
 
-function drawRevealStroke(
+function drawDarknessStroke(
   ctx: CanvasRenderingContext2D,
-  stroke: SceneDarknessRevealStroke | { points: { x: number; y: number }[]; radiusN: number },
+  stroke: SceneDarknessRevealStroke | { points: { x: number; y: number }[]; radiusN: number; mode?: SceneDarknessStrokeMode },
   w: number,
   h: number,
 ): void {
   const pts = stroke.points;
   if (pts.length === 0) return;
+  const mode = normalizeSceneDarknessStrokeMode(stroke.mode);
+  ctx.globalCompositeOperation = mode === 'cover' ? 'source-over' : 'destination-out';
+  ctx.fillStyle = '#000000';
+  ctx.strokeStyle = '#000000';
   const r = stroke.radiusN * Math.min(w, h);
   if (pts.length === 1) {
     const p = pts[0];
@@ -30,7 +39,6 @@ function drawRevealStroke(
   ctx.lineCap = 'round';
   ctx.lineJoin = 'round';
   ctx.lineWidth = r * 2;
-  ctx.strokeStyle = 'rgba(0,0,0,1)';
   ctx.beginPath();
   const first = pts[0];
   if (!first) return;
@@ -65,12 +73,11 @@ export function SceneDarknessOverlay({
     ctx.fillStyle = '#000000';
     ctx.fillRect(0, 0, w, h);
 
-    ctx.globalCompositeOperation = 'destination-out';
     for (const stroke of state.strokes) {
-      drawRevealStroke(ctx, stroke, w, h);
+      drawDarknessStroke(ctx, stroke, w, h);
     }
     if (state.draft && state.draft.points.length > 0) {
-      drawRevealStroke(ctx, state.draft, w, h);
+      drawDarknessStroke(ctx, state.draft, w, h);
     }
   }, [state, viewport]);