Skip unnecessary longitude normalization on antimeridian-crossing box

[tinko92]

The program in issue #1452 triggers some cases in which a box shrinks when being expanded with a point that is already within it, due to rounding errors introduced unnecessarily in coordinate normalization. When a box crosses the antimeridian, the longitude of its max point is first normalized to be within [-180, 180], then increased by half_period() again, which can introduce rounding error, including shrinking the box. the shrinking then seems to produce unstable behaviour in the rtree when the computed area difference between a boxes expansion and itself is negative. This is only a small part of the tree instability in #1452 and does not resolve most of the issue but isolated enough from the other issues to be treated in a separated PR.

This PR contains the following changes:

1. Adds a flag that detects when a box crosses the antimeridian and has otherwise valid max longitude, and passes this flag down to coordinate normalization, so that this longitude is then not normalized down.
2. Add a test case with tolerance=0.0, in which the box would shrink without this change.
3. Update the box_equal code to respect the passed tolerance in longitude comparison rather than using the (more lenient) bg::math::equals unconditionally.

[Copilot]

Pull request overview

Addresses a rounding-error edge case where expanding a spheroidal box that crosses the antimeridian can unexpectedly shrink due to unnecessary longitude normalization.

Changes:

• Adds an “allow antimeridian crossing” path to longitude normalization to avoid lossy normalize-then-shift behavior.
• Updates spheroidal box normalization to pass the new flag when appropriate.
• Adds a regression test for the shrink-on-expand scenario and tightens box equality checks to respect the provided tolerance.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
test/algorithms/envelope_expand/test_envelope_expand_on_spheroid.hpp	Adjusts box equality logic for longitude comparisons under tolerance.
test/algorithms/envelope_expand/expand_on_spheroid.cpp	Adds a regression test for antimeridian-crossing box expansion stability.
include/boost/geometry/util/normalize_spheroidal_coordinates.hpp	Extends longitude normalization with an option to skip normalization for antimeridian-crossing representation.
include/boost/geometry/util/normalize_spheroidal_box_coordinates.hpp	Detects the special antimeridian-crossing representation and forwards the skip-normalization flag.

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[barendgehrels]

Thanks for the fix! I'm OK