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/render/live_ink_painter.dart

@@ -39,13 +39,7 @@ class LiveInkPainter extends CustomPainter {
         isComplete: false, thinning: thinning);
     if (path.getBounds().isEmpty) return;
 
-    canvas.drawPath(
-      path,
-      Paint()
-        ..color = Color(stroke.color)
-        ..style = PaintingStyle.fill
-        ..isAntiAlias = true,
-    );
+    canvas.drawPath(path, paintForEditorStroke(stroke));
   }
 
   @override