feat(eval): overhaul HTML viewer with leaderboard and stats (#491)

Add a leaderboard initial view with score bar graphs, model selection
for comparison, and click-to-detail. Include inference statistics
(latency, cost, tokens) in JSON output and display them in the
comparison grid with popup for full breakdown. Parse model:effort@provider
filenames for multi-line headers, show score and group breakdown as
dedicated rows.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: owtaylor