Make real-screen candidate comparison a keyboard choice

[aruintelligence]

Change

• Express the two mutually exclusive captured-screen candidates as a named radio group with one selected choice.
• Add arrow, Home, and End navigation with roving tab focus. The visible continuity counts and outcome update with the selected candidate.
• Test keyboard movement, selected state, tab stop, and the result text in a focused DOM harness.

Verification

• node --test test/home-boundary-selection.test.js test/home-challenge-selection.test.js test/home-field-choice.test.js (3 passed locally).
• Inspect the live homepage after Pages deployment.

The comparison uses project-authored candidates over two hashed desktop captures. It does not change the production subject site or measure visitor outcomes.