feat(pen): brush opacity + highlighter multiply

Honor each brush's opacity/blend so the brushes feel distinct
(closes TODO(brush-opacity)).

- Shared paint resolver: a stroke's color alpha is multiplied by its
  brush opacity; ballpoint/pencil opacity is tied to pressure
  (per-stroke average this increment) so a ballpoint reads lighter
  than a solid fountain pen.
- Highlighter paints with BlendMode.multiply and draws once, so
  cross-stroke overlap darkens like a real marker while self-overlap
  doesn't.
- Applied across BOTH render paths (PenCanvas static/live painters and
  the PDF _PageOverlayPainter).

Pencil paper-grain texture still deferred (TODO brush-texture); brush
kind is not yet serialized (TODO brush-persist — next). analyze clean,
tests green.

lib/editor/canvas/ink_painters.dart

@@ -65,6 +65,34 @@ Path buildStrokePath(
   return path;
 }
 
+/// Mean point pressure (`pressure ?? 0.5`) of a [PenStroke], for the per-stroke
+/// opacity resolution (spec §3/§4 tie ballpoint/pencil opacity to pressure).
+double _avgPressure(PenStroke stroke) {
+  if (stroke.points.isEmpty) return 0.5;
+  var sum = 0.0;
+  for (final p in stroke.points) {
+    sum += p.pressure ?? 0.5;
+  }
+  return sum / stroke.points.length;
+}
+
+/// THE single fill [Paint] for a committed/live stroke, with the brush's
+/// resolved opacity (multiplied into the color's alpha) and blend mode applied
+/// — closes TODO(brush-opacity). Shared by [StaticInkPainter]/[LiveInkPainter]
+/// and the PDF overlay painter so both render paths composite identically.
+Paint paintForStroke(PenStroke stroke) {
+  final resolved = resolveStrokePaint(
+    stroke.brush,
+    stroke.color,
+    pressureAvg: _avgPressure(stroke),
+  );
+  return Paint()
+    ..color = resolved.color
+    ..blendMode = resolved.blendMode
+    ..style = PaintingStyle.fill
+    ..isAntiAlias = true;
+}
+
 /// Paints all committed strokes for the page. Repaints only when the stroke
 /// list identity or page size changes (kept behind a RepaintBoundary).
 class StaticInkPainter extends CustomPainter {
@@ -86,13 +114,9 @@ class StaticInkPainter extends CustomPainter {
       final path =
           buildStrokePath(stroke, pageSize, isComplete: true, thinning: thinning);
       if (path.getBounds().isEmpty) continue;
-      canvas.drawPath(
-        path,
-        Paint()
-          ..color = Color(stroke.color)
-          ..style = PaintingStyle.fill
-          ..isAntiAlias = true,
-      );
+      // Single drawPath per stroke ⇒ a highlighter's own self-overlap never
+      // darkens; cross-stroke overlap darkens via BlendMode.multiply (marker).
+      canvas.drawPath(path, paintForStroke(stroke));
     }
   }
 
@@ -268,13 +292,7 @@ class LiveInkPainter extends CustomPainter {
     final path =
         buildStrokePath(s, pageSize, isComplete: false, thinning: thinning);
     if (path.getBounds().isEmpty) return;
-    canvas.drawPath(
-      path,
-      Paint()
-        ..color = Color(s.color)
-        ..style = PaintingStyle.fill
-        ..isAntiAlias = true,
-    );
+    canvas.drawPath(path, paintForStroke(s));
   }
 
   @override