fix: soft-clamp pinch zoom and Krita-inspired brush opacity

Hard SDROP avalanches froze lastRaw while zoom still crawled; soft-clamp
and re-anchor instead. Ballpoint is near-solid, pencil uses soft √p without
multiply stacking; PDF ink falls back to nearest page during zoom settle.

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

test/brush_test.dart

@@ -57,12 +57,13 @@ void main() {
       expect(b.opacity, lessThan(1.0));
     });
 
-    test('pencil — Sqrt (√p), moderate thinning 0.5, scratchy streamline', () {
+    test('pencil — Sqrt (√p), moderate thinning, soft streamline', () {
       final b = brushProfileFor(BrushKind.pencil);
       expect(b.pressureGamma, 0.5); // rnote Sqrt / √p
-      expect(b.pfThinning, 0.5);
-      expect(b.pfStreamline, 0.4);
+      expect(b.pfThinning, 0.45);
+      expect(b.pfStreamline, 0.35);
       expect(b.taper, isFalse);
+      expect(b.blendMultiply, isFalse);
     });
   });