feat(proxy/store): show spend and attribution in the learn report

Render the window cost, the effective input rate after cache reuse, and the
per-sink spend figures in the static HTML report, and carry each confirmed row's
attribution method next to its number so the report never shows a saving without
saying how it was measured.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>