Raster parent tile fallback

While ordinary shared raster sources wait for exact tiles after zooming, the map can display the nearest decoded ancestor from the same source and generation. Each parent is cropped to the target tile footprint. Exact tiles replace it on arrival, with opacity applied once. Existing addSource/addLayer imagery uses this automatically.

const info = await map.sqs.getSourceInfo('imagery');
console.log(info.tileState.fallbackTiles);
console.log(info.tileState.readyTiles, info.loaded);

fallbackTiles counts displayed fallback instances. readyTiles counts exact current tiles only; fallback does not make loaded true. Derived RGBA shares the map's64MiB decoded tile budget and existing update/cancellation/generation rules. Areas without a cached ancestor or enough budget remain empty.

The complete HTML includes all four buttons, events, embedded images and request protocol, and runs without an acceptance server. The portable script uses the acceptance server to exercise the same native/main-thread/Worker semantics. SqsMap generated the example images. Real imagery must retain provider-required attribution and copyright.

Only nearest-parent fallback for ordinary raster is implemented. Descendant fallback, DEM/MVT fallback, cross-tile fades and stitching remain pending. Mobile devices and hardware require separate acceptance.