define image order

The graphic indicates that newer images should be places earlier in the tiles array, with older images placed later in the tiles array, but this is never specified in the spec.

If downstream applications make assumptions about what the order of the files in the array represents, we should document that in the spec.

Start by locating the mosaic specification and the section defining the tiles array. Review how image ordering is currently described, then document whether newer images belong earlier and older images later, so downstream applications have an explicit ordering rule.

Written by the indexing model from the issue text.