Fix sampling/mask robustness and add regression tests

[clemsgrs]

Summary

• fix independent sampling crash when visualization is disabled
• subtract all contour holes when precomputing tissue masks
• replace silent tile-filter exceptions with summarized warnings while keeping tiles
• add regression tests for all three behaviors

Testing

• /Users/clems/Code/venv/hs2p/bin/python -m pytest -q /Users/clems/Code/hs2p/tests
  • 5 passed, 1 skipped