Run antimeridian_split through apply on the polygonal traits

[asinghvi17]

antimeridian_split walked its input with a bespoke _each_polygon helper that
only knew how to handle a bare Polygon or MultiPolygon. This replaces that
with apply on the polygonal trait target:

const _AM_TARGETS = TraitTarget{Union{GI.PolygonTrait, GI.MultiPolygonTrait}}()

apply(WithTrait((trait, g) -> _split_geom(trait, g, λn, pole_spacing)), _AM_TARGETS, work; threaded)

so the split now runs on every polygonal geometry in any nesting apply
understands — features, feature collections, tables, (nested) vectors — and the
structure above the polygons is rebuilt around the results. A bare Polygon
still comes back as a MultiPolygon, as before.

Notes

• The target is the union, not PolygonTrait alone. A MultiPolygon must
  split part-by-part into one flat piece list; targeting only PolygonTrait
  would rebuild it as a MultiPolygon of MultiPolygons. So there are two
  target methods, _split_geom(::GI.PolygonTrait, …) and
  _split_geom(::GI.MultiPolygonTrait, …), both feeding the unchanged
  _split_polygon!.
• The ArgumentError guard stays, but shrank. apply never hands a
  LineString to f when the target is polygonal — it descends toward points
  instead — so a bare unsupported geometry still has to be rejected up front.
  _check_split_trait now rejects only geometry traits that aren't polygonal,
  letting nothing/feature traits fall through to apply's own descent.
• The rotated-pole step stays outside the apply. _rotate_to_pole is
  itself an apply(PointTrait), so it already handles any nesting, and doing it
  once avoids rebuilding the rotation matrix per polygon.
• New threaded keyword, forwarded to apply.

No change to the split algorithm itself — the arrangement, face selection,
branch fixing and pole-row emission are untouched.

Test plan

test/transformations/antimeridian_split.jl passes in full, including the
Natural Earth cases. New nested inputs testset covers vector input (threaded
and not), a feature collection with properties preserved, and MultiPolygon
input — the last of which was previously untested.

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