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/engine/stroke_geometry.dart

@@ -88,10 +88,9 @@ List<Offset> freehandOutlinePoints({
 
 /// Build perfect_freehand [pf.StrokeOptions] from a [BrushProfile] (spec §4).
 ///
-/// TODO(brush-opacity): [BrushProfile.opacity] / [BrushProfile.blendMultiply]
-/// are NOT consumed here — geometry only this increment. The caller still paints
-/// fill color (with its own alpha) and BlendMode.srcOver; per-stroke opacity /
-/// BlendMode.multiply for ballpoint/pencil/highlighter lands later.
+/// Geometry only: [BrushProfile.opacity] / [BrushProfile.blendMultiply] are
+/// consumed by the painters' [paintForEditorStroke] / `paintForStroke` (via
+/// [resolveStrokePaint]), NOT here — this stays a pure outline recipe.
 pf.StrokeOptions _optionsFromBrush(
   BrushProfile brush, {
   required double size,
@@ -171,3 +170,36 @@ Path buildStrokeOutline(
   path.close();
   return path;
 }
+
+/// Mean point pressure (`pressure ?? 0.5`) of an [EditorStroke], for the
+/// per-stroke opacity resolution (spec §3/§4 tie ballpoint/pencil opacity to
+/// pressure).
+double _avgPressure(EditorStroke 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 an [EditorStroke], with the brush's resolved
+/// opacity (multiplied into the color's alpha) and blend mode applied — closes
+/// TODO(brush-opacity). Shared by the committed [Picture] and live painters so
+/// the EditorStroke render path composites exactly like the PenStroke one.
+///
+/// The stroke is drawn as ONE fill polygon, so a highlighter's own self-overlap
+/// never darkens; cross-stroke overlap darkens via [BlendMode.multiply]
+/// (marker build-up). TODO(brush-texture): pencil paper grain still deferred.
+Paint paintForEditorStroke(EditorStroke stroke) {
+  final resolved = resolveStrokePaint(
+    stroke.brush,
+    stroke.color,
+    pressureAvg: _avgPressure(stroke),
+  );
+  return Paint()
+    ..color = resolved.color
+    ..blendMode = resolved.blendMode
+    ..style = PaintingStyle.fill
+    ..isAntiAlias = true;
+}