Enable `symbol-sort-key` when text/icons on a layer have opposing overlap rules

Nobody has claimed this yet.

When `icon-allow-overlap: true` and `text-allow-overlap: false` or vice-versa for a symbol layer, there is no way to sort icons + text consistently with [`symbol-sort-key`](https://docs.mapbox.com/mapbox-gl-js/style-spec/layers/#layout-symbol-symbol-sort-key).

Sort key determines draw order in the renderer. Features with lower sort keys are drawn and placed first.
- When overlap is enabled, features with higher sort keys can be drawn and placed over previous features, so features with higher sort keys have higher priority.
- When overlap is disabled, features with higher sort keys cannot be placed over previous features because of the way collision detection works. Therefore, during placement, features with _lower_ sort keys have higher priority.

This behavior has always existed in the renderer and it won't be straightforward to change.

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Start by tracing the symbol-sort-key style-spec entry and the renderer's placement and collision behavior described in the issue. Determine how opposing icon and text overlap rules should share a consistent ordering, then verify that the behavior is covered for both overlap directions. Done means symbol-sort-key provides predictable priority in these cases without regressing existing placement behavior.

Written by the indexing model from the issue text.