fix: pointer switch jump on ACTION_POINTER_UP

[VoidK2]

Summary

Fixes one-frame jump when releasing one finger during pinch zoom.

Problem

When performing a pinch zoom and releasing one finger (pointer transition from 2 to 1), the image center appears to "jump" for one frame. This issue is device-dependent and more visible on certain OEM devices with specific touch sampling rates and motion event batching strategies.

Root Cause

When ACTION_POINTER_UP switches activePointerId, the first ACTION_MOVE frame uses stale lastTouch coordinates from the two-finger phase to calculate panChange, causing a one-frame jump in the pan delta.

Solution

• Added lastPointCount tracking to detect pointer count changes (e.g., 2→1 or 1→2)
• On first frame after pointer count change:
  • Reset drag baselines (firstTouch, lastTouch, lastGestureFocus)
  • Zero out panChange to prevent the jump
• This ensures clean state transition between single-finger and multi-finger gesture phases

Changes

File modified: zoomimage-view/src/main/kotlin/com/github/panpf/zoomimage/view/zoom/internal/ScaleDragGestureDetector.kt

Key changes:

1. Added lastPointCount field to track pointer count transitions
2. Detect pointCountChanged in ACTION_MOVE
3. On transition: reset baselines and zero panChange

Testing

• ✅ Two-finger zoom → release one finger while still moving
• ✅ One-finger drag → add second finger to zoom
• ✅ Zoom past bounds → release → rollback center check
• ✅ No regression in fling/rollback behavior
• ✅ ViewPager interception behavior unchanged

Related Issues

This is related to the pointer-count transition handling that was previously addressed in issue #12 (fixed in v1.0.1), and later architectural changes in commit 605b4ac1 ("supports two-finger drag gestures") touched the same detector area.