fix: preserve device colors for explicit palette entries

[benletchford]

Summary

• keep the active device ColorTable entry unchanged whenever pmExplicit is present
• apply explicit index semantics when usage also includes pmTolerant
• update the activation regression test for mixed explicit usage

Validation

• cargo fmt --all -- --check
• cargo test activatepalette_preserves_device_rgb_for_mixed_explicit_entries
• Civilization II startup dialogs, menus, and explored terrain recover their intended colors

References

• Apple, Inside Macintosh: Advanced Color Imaging (1995), pp. 1-12–1-13
• Progresses Civilization II first map renders placeholder terrain tiles #600; the unexplored-terrain mask artifact remains separately reproducible.