Restrict visualization overlays to explicitly colored annotations

[clemsgrs]

Summary

• change mask overlay semantics so only annotations with non-null color_mapping are blended
• apply the same strict behavior to both tile overlays and slide overlays
• thread pixel_mapping and color_mapping through sampling visualization calls and validate mapping shape/keys
• add regression tests ensuring uncolored labels remain untouched (no black/dark overlay)

Details

• introduced a shared _build_overlay_alpha helper in hs2p/wsi/__init__.py
• updated overlay_mask_on_tile and overlay_mask_on_slide to use strict colored-label selection
• updated visualization call chain (visualize_coordinates / draw_grid_from_coordinates) to pass mapping context
• updated sampling visualization code to validate and forward mappings

Testing

• /Users/clems/Code/venv/hs2p/bin/python -m pytest tests/test_overlay_semantics.py -q
• /Users/clems/Code/venv/hs2p/bin/python -m pytest tests -q