Show Greatest inclusion reasons on catalog tiles

docs/greatest.md

@@ -1,6 +1,7 @@
 # Shijō Saikō no Manga — Greatest of All Time
 
 `greatest.html` is a second, single-page dashboard. The Japanese title is **史上最高の漫画**, romanized **Shijō Saikō no Manga** (“greatest manga of all time”). It displays the [research reading map](manga-reading-map-2026-10-09.md) as ranked five-year cohorts from before 1970 through 2025–2026. Each period keeps its own separator and tile grid.
+Each tile shows a four-line **Why it matters** preview from the editorial rationale; opening the tile shows the complete text under the same heading. The note is a reason for inclusion, not a plot synopsis or a machine-generated rating.
 
 ## Separation and data ownership