fetchAndCacheTile

Bulk-download path: by default skip already-cached tiles, otherwise force a network fetch and persist. Bypasses isCacheOnly — bulk is user-initiated and orthogonal to the renderer's offline toggle. Returns true on cache hit / successful fetch + write, false on cache miss + (no network / fetch failure / decode failure). When overrideCache is true, the cache hit short-circuit is skipped and every tile is re-downloaded.