fix(zoom): stop re-baseline scale oscillation

Device log showed the applied scale oscillating ~1.4x every frame while
the raw pinch was smooth (cur 1.116->0.797->1.074, raw ~0.46). Root cause:
on a pointer-count re-baseline (Windows touch flickers 2<->1<->2 mid-pinch)
the code set _scaleStart = matrix.getMaxScaleOnAxis() — a read-back captured
at a glitchy instant — so the absolute map K = scaleStart/rawScaleAtBaseline
jumped frame to frame.

Fix: anchor the re-baseline to the CLEAN tracked _lastAppliedScale instead
of the live matrix read-back, so the displayed scale is continuous across
the re-baseline regardless of any matrix transient. The math is already
covered by the pinch_scale_solver "same-instant re-baseline" test; this just
feeds it the right value.

flutter analyze: 0. pinch_scale_solver + pen_zoom: pass.

lib/editor/canvas/pen_interactive_viewer.dart

@@ -194,13 +194,20 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
     // The transitional frame itself is skipped.
     if (details.pointerCount != _lastPointerCount) {
       _lastPointerCount = details.pointerCount;
-      _scaleStart = _transformer.value.getMaxScaleOnAxis();
+      // Anchor the new baseline to the CLEAN tracked scale (_lastAppliedScale),
+      // NOT a fresh matrix read-back. Windows touch flickers the pointer count
+      // (2↔1↔2) mid-pinch, firing this re-baseline spuriously; reading
+      // getMaxScaleOnAxis() at that glitchy instant popped _scaleStart to a
+      // noisy value, so the absolute map K = scaleStart / rawScaleAtBaseline
+      // oscillated frame-to-frame (the reported "zoom jump"). Using
+      // _lastAppliedScale makes the displayed scale CONTINUOUS across the
+      // re-baseline: target == _lastAppliedScale at this instant, regardless of
+      // any transient in the live matrix.
+      _scaleStart = _lastAppliedScale;
       _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
       _lastRawScale = details.scale;
-      _lastAppliedScale = _scaleStart!;
-      // Re-anchor the absolute mapping: from here, cumulative scale is measured
-      // relative to THIS frame's details.scale (so the next good frame starts
-      // from _scaleStart, not _scaleStart * a stale cumulative value).
+      // Re-anchor the cumulative scale to THIS frame's details.scale so the next
+      // good frame resumes from _scaleStart (not _scaleStart × a stale ratio).
       _rawScaleAtBaseline = details.scale;
       InputDiagnostics.instance.recordRebaseline();
       return;