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>

lib/editor/canvas/pinch_scale_solver.dart

@@ -18,6 +18,12 @@
 // pinch has accumulated (e.g. 0.40) — multiplying the fresh `scaleStart` by
 // that stale 0.40 popped the zoom to a wrong scale and snapped back (the
 // reported flicker). Normalizing against `rawScaleAtBaseline` removes the pop.
+//
+// Soft-clamp (Surface 2026-08-05 diag): a HARD drop of frames whose per-step
+// ratio exceeds the glitch band caused an avalanche — lastRaw never advanced,
+// so every subsequent frame also dropped while pdfrx/live zoom still crawled.
+// [softClampedPinchStep] always returns an applied scale, clamping the step,
+// and tells the caller to re-anchor when a spike was clipped.
 
 import 'package:flutter/foundation.dart' show clampDouble;
 
@@ -39,3 +45,59 @@ double absolutePinchScale({
       rawScaleAtBaseline > 0 ? rawScale / rawScaleAtBaseline : 1.0;
   return clampDouble(scaleStart * cumulative, minScale, maxScale);
 }
+
+/// Result of one soft-clamped pinch step.
+class SoftPinchStep {
+  const SoftPinchStep({
+    required this.appliedScale,
+    required this.reanchor,
+    required this.spiked,
+  });
+
+  /// Scale to write into the matrix / controller this frame.
+  final double appliedScale;
+
+  /// When true the caller must set `scaleStart = appliedScale` and
+  /// `rawScaleAtBaseline = rawScale` so absolute tracking does not keep
+  /// fighting the clamp on later frames.
+  final bool reanchor;
+
+  /// True when the ideal absolute target was clipped by the per-step band.
+  final bool spiked;
+}
+
+/// Soft-clamp the per-frame scale change instead of dropping the frame.
+///
+/// Ideal scale comes from [absolutePinchScale]. The step from
+/// [lastAppliedScale] is then limited to `[1/maxStepRatio, maxStepRatio]`.
+/// Spikes still get partially applied (smooth catch-up) and the caller
+/// re-anchors so the next frame starts clean.
+SoftPinchStep softClampedPinchStep({
+  required double scaleStart,
+  required double rawScaleAtBaseline,
+  required double rawScale,
+  required double lastAppliedScale,
+  required double minScale,
+  required double maxScale,
+  required double maxStepRatio,
+}) {
+  final ideal = absolutePinchScale(
+    scaleStart: scaleStart,
+    rawScaleAtBaseline: rawScaleAtBaseline,
+    rawScale: rawScale,
+    minScale: minScale,
+    maxScale: maxScale,
+  );
+  if (lastAppliedScale <= 0 || maxStepRatio <= 1.0) {
+    return SoftPinchStep(appliedScale: ideal, reanchor: false, spiked: false);
+  }
+  final lo = lastAppliedScale / maxStepRatio;
+  final hi = lastAppliedScale * maxStepRatio;
+  final applied = clampDouble(ideal, lo, hi);
+  final spiked = applied != ideal;
+  return SoftPinchStep(
+    appliedScale: clampDouble(applied, minScale, maxScale),
+    reanchor: spiked,
+    spiked: spiked,
+  );
+}