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.
Preserve page-backed large-catalog import while integrating upstream catalog and documentation changes. Adapt only the sparse test fixture to the shared upstream fixture helper.
Use the equivalent complete RGB-triple iterator required by Rust 1.99 Clippy; the same lint failure was reproduced on the exact upstream baseline.
Validation: 47 catalog tests, nine spot tests, exact sparse regression, all eight CI gates, and an original synthetic headless import/selection check passed.
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