perf: reduce lag when dragging large selected stroke groups

[markm39]

Problem

Dragging a large selected stroke group can drop FPS and feel noticeably behind the pointer. This is most visible when many strokes are selected and the user drags the selection around the page.

Expected behavior

Selected stroke groups should stay interactive during drag, even for large selections. The transform preview should track the pointer smoothly, and commit work should happen after the gesture instead of during every move when possible.

Notes

Possible directions:

• Move more selection transform preview work native-side.
• Batch selected-stroke updates during drag.
• Defer expensive serialization/export work until the drag ends.
• Add a stress test with a large selected stroke group.

Acceptance criteria

• Dragging a large stroke selection no longer causes obvious frame drops.
• Selection state remains correct after drag commit.
• Existing selection/serialization tests still pass.