Loader skewer selection

[haticekaratay]

Description

This PR adds a new skewer selection mode for interactive footprint selection in the loaders infrastructure. The skewer mode selects footprints by evaluating whether a clicked sky coordinate lies within a footprint’s spherical polygon, and is available in addition to the existing nearest-edge footprint selection behavior.

This work was initially developed for the Footprints plugin (see #3794), but has since been refactored and integrated into the loaders infrastructure. By implementing this in loaders, the skewer selection mode is now available wherever loader-based footprint selection is used.

Screen.Recording.2026-01-20.at.1.55.09.PM.mov

To make it easier to visualize how the skewer selection mode works, check the demo with well-separated footprints.
When clicking inside overlapping footprints, the skewer mode automatically selects the one with the smallest polygon area. This ensures that when a smaller footprint is nested within a larger one, clicking in the overlap region will correctly select the more specific (smaller) footprint.

Screen.Recording.2026-01-20.at.3.06.50.PM.mov

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

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• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?