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open suspend override fun tryReadCachedTile(z: Int, x: Int, y: Int, sector: Sector): Flow<CachedFeatureRow>?
Cache-only fast-path read for (z, x, y). Default returns null — a plain network source has no cache to consult. Cache-backed sources override to read their store without going through the network-fetch concurrency budget. Empty flow is the negative-cache sentinel ("fetched, no features"); null is a true miss.
sector is the tile's geographic bounds — supplied so a cache-backed source can issue a stale-while-revalidate background refetch (which needs the bbox) without a second coordinate→sector conversion on the caller's side. Read-only hits ignore it.