Speed up multi-ROI deletion in the GUI

[chenyenchung]

This a proof-of-concept change associated with #1419 .

• Add batch deletion utilities in cellpose/gui/delete_utils.py to normalize ROI IDs and relabel masks in a single pass.
• Update cellpose/gui/gui.py so multi-ROI deletion uses the batch path, while single-ROI deletion remains the same.
• Preserve existing GUI-side effects, including ROI count updates, saved-set updates, and deletion tracking.
• Add regression tests in tests/test_gui_delete_utils.py to verify that the batch path behaves in the same way as the original deletion behavior in both 2D and 3D cases.
• Treat invalid or duplicate ROI IDs as no-ops.

Testing

• Added a fixed-case regression test.
• Added randomized 2D and 3D equivalence tests against the legacy deletion path.
• Added a no-op test for invalid ROI IDs.

[chenyenchung]

Rebased the commit to the current main.