Make homepage prediction challenge a keyboard choice

[aruintelligence]

Change

• Present the controlled-change and prediction controls as named one-choice radio groups.
• Support arrow keys, Home, and End with one tab stop in each group; reset the prediction when the controlled change switches.
• Preserve the existing result and checker links, and test selection, wraparound, feedback, and reset in a focused DOM harness.

Verification

• node --test test/home-challenge-selection.test.js test/home-boundary-selection.test.js (2 passed locally).
• Live Pages interaction after merge.

This branch starts at #323 and should merge after that PR. The challenge is a project-authored prediction over saved local evidence, not a provider or human approval.