find Cached Vector Tile For Nominal Zxy
fun findCachedVectorTileForNominalZxy(z: Int, x: Int, y: Int, allowStaleTimeFallback: Boolean = true): Tile<VectorData>?
Finds a cached vector tile by nominal z/x/y. Tries the validTime key first, then scans the cache so draws still resolve tiles after subtle key drift (e.g. time format rounding).