[jpenilla]

Coarser zoom levels were point-sampled from each region's new pixels, taking the top-left block of every 2^k square, which made thin features break up and shading alias when zoomed out. Each level is now built from the finished tile one level finer with a 2×2 box filter in linear light. Colors are weighted by alpha and alpha is set to coverage, so edges against void or unrendered areas fade out instead of darkening. Because the whole finer tile is used, areas rendered earlier are included.

Both are zoom 0 (8 blocks per pixel) at native size from the demo world. Before is the demo site's existing tiles, after is zoom 0 rebuilt from the same max zoom tiles.

Saving

A region's save is now a chain: its max zoom tile is patched and written, then each parent is read, the written child is downsampled into its quadrant, and the parent is written, up to zoom 0. A failed write ends that region's chain.

Updates to the same tile run one at a time in the order they became ready, using a new KeyedSerialExecutor that queues waiting tasks per key instead of holding a thread, so only real dependencies are waited on. A parent update is queued before its child's tile is released, so updates to a shared parent run in the order their sources were written.

Tile file reading and writing moved from RegionImage to TileImages, leaving RegionImage with only the region's pixels. Queue capacity and shutdown use one lock and condition: shutdown rejects new saves, waits for in-flight chains, including parent updates they queue while finishing, then stops the workers. Checkpoints still count a region as done once queued, and max-pending-images still limits whole region saves.

Upgrading

Existing coarse tiles keep their point-sampled content until a region under them is saved again. A full render updates them all.