Keep main's spots.rs implementation and preserve the catalog reader and
updated capability documentation. Readers are byte-identical to the PR.
Validation: 48 catalog and 9 spot tests pass on candidate and main.
Formatting, Clippy, parity, layers, assets and WASM checks pass.
Full CI remains blocked by the same mask-overlay test failure reproduced
on main; no assertions or tests were changed to hide it.
Refs #798.
FredApps
6f12b28f28