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/pinch_scale_solver_test.dart

@@ -61,4 +61,62 @@ void main() {
   test('degenerate baseline (0) is treated as no cumulative change', () {
     expect(solve(2.0, 0.0, 5.0), closeTo(2.0, 1e-9));
   });
+
+  group('softClampedPinchStep', () {
+    test('normal step passes through unchanged', () {
+      final s = softClampedPinchStep(
+        scaleStart: 1.0,
+        rawScaleAtBaseline: 1.0,
+        rawScale: 1.1,
+        lastAppliedScale: 1.0,
+        minScale: min,
+        maxScale: max,
+        maxStepRatio: 1.18,
+      );
+      expect(s.appliedScale, closeTo(1.1, 1e-9));
+      expect(s.spiked, isFalse);
+      expect(s.reanchor, isFalse);
+    });
+
+    test('spike is soft-clamped and requests reanchor (no hard drop)', () {
+      // Ideal would jump 1.0 → 2.0 (ratio 2.0 >> 1.18).
+      final s = softClampedPinchStep(
+        scaleStart: 1.0,
+        rawScaleAtBaseline: 1.0,
+        rawScale: 2.0,
+        lastAppliedScale: 1.0,
+        minScale: min,
+        maxScale: max,
+        maxStepRatio: 1.18,
+      );
+      expect(s.appliedScale, closeTo(1.18, 1e-9));
+      expect(s.spiked, isTrue);
+      expect(s.reanchor, isTrue);
+    });
+
+    test('after reanchor, next frame can continue smoothly', () {
+      final spike = softClampedPinchStep(
+        scaleStart: 1.0,
+        rawScaleAtBaseline: 1.0,
+        rawScale: 2.0,
+        lastAppliedScale: 1.0,
+        minScale: min,
+        maxScale: max,
+        maxStepRatio: 1.18,
+      );
+      // Caller re-anchors: scaleStart=1.18, baseline=2.0, last=1.18.
+      // Continue toward ideal 2.0 with raw still 2.0 → applied stays 1.18.
+      final next = softClampedPinchStep(
+        scaleStart: spike.appliedScale,
+        rawScaleAtBaseline: 2.0,
+        rawScale: 2.0,
+        lastAppliedScale: spike.appliedScale,
+        minScale: min,
+        maxScale: max,
+        maxStepRatio: 1.18,
+      );
+      expect(next.appliedScale, closeTo(1.18, 1e-9));
+      expect(next.spiked, isFalse);
+    });
+  });
 }