fix(cli): consolidate layout and contrast audit correctness

[miguel-heygen]

Summary

• make occlusion checks respect transparent image pixels and contained-image letterboxing
• evaluate text strokes and transparent backdrops correctly in contrast audits
• honor explicit overflow opt-outs in both runtime diagnostics and validation
• preserve all regression coverage from fix(cli): respect transparent image pixels in occlusion audit #2367, fix(cli): account for text strokes in contrast checks #2349, fix(cli): honor text overflow opt-outs #2336, and fix(cli): skip contrast on transparent backdrops #2313

Verification

• full workspace build
• CLI test suite: 1,738 passed, 2 skipped
• CLI typecheck
• focused layout audit suite: 64 passed
• generated skills manifest refreshed