README: document the geo/cells_v1 manifest and what it does for warm reloads

README.md

@@ -213,10 +213,15 @@ Everything is stored in IndexedDB, database `background-tiles`, store `tiles`:
 | `geo/roadchains_v1` | merged road polylines (rebuilt only when a cell is re-fetched) |
 | `geo/placecache_v1` | decoded settlement list |
 | `geo/airports_v1` | airfield list used by the aviation layer |
+| `geo/cells_v1` | manifest: every cached Overpass cell with the time it was stored |
 
 The star catalogue is not cached: `build.py` inlines `vendor/stars.json` into
 `window.__starsData`. Weather is the only thing fetched live on every load.
 
+The manifest is what makes a warm reload fast: the road and settlement layers
+compare `geo/cells_v1` against their build records and replay the finished
+geometry without reading a single raw Overpass cell.
+
 Cached data is never re-fetched unless `CONFIG.tiles.expireDays` is set. To
 start clean, delete the `background-tiles` database in the browser.