AI Recommendation Index — archive dates, dated rankings, and the change log

How this category's AI recommendations changed over time. Returns every date the category has an archived snapshot for (newest first, with earliest/latest bounds — use them for /api/index/{slug}/as-of/{date}), the recent dated rankings trimmed to the top band, and a change log computed by diffing adjacent snapshots at read time: who entered the ranking, who left it, rank moves, and share moves that clear the aggregator's flat band (sub-noise moves are deliberately omitted). Each step also flags an extraction_version change between the two dates, so a methodology shift never reads as a silent reshuffle.