🧪 [testing improvement] Add direct APCA calculation tests

[immineal]

🎯 What: Added direct mathematical validation tests for the calcAPCA function in the color contrast tool.
📊 Coverage: Tests directly evaluate pure input/output math for extreme boundary scenarios (Black/White, White/Black) and generic scenarios (Red/White, Green/White, Same/Same), bypassing DOM interaction.
✨ Result: Test suite is now robust against silent precision/calculation regressions in the underlying APCA accessibility math.

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PR created automatically by Jules for task 8961763344793507981 started by @immineal

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