OverlayNG: Merge linear outputs

[dbaston]

As discussed in https://lists.osgeo.org/pipermail/geos-devel/2026-May/011167.html (May) and https://lists.osgeo.org/pipermail/geos-devel/2026-June/011179.html (June)

I have not modified the XML tests yet.

[dbaston]

I've updated the XML tests to expect merged output, with one exception. test_split.xml intends to show how a side effect of Geometry::difference allows splitting a linear geometry with a second linear geometry. Since this PR removes that side effect, I removed the test file.

[dbaston]

It appears PostGIS is relying on the splitting side-effect of GEOSDifference:

https://github.com/postgis/postgis/blob/423a5fd79c2e4d2a1fd88911ffbfb9f906d4a6a8/liblwgeom/lwgeom_geos_split.c#L115

So PostGIS would need to be updated to use GEOSSplit in this case instead.

[pramsey]

They are in overlay, so they should be in this operation too, I think is consistent. Probably better than unpredictable ordering based first-in / last-in selection.

[dbaston]

Agree that averaging is desirable (if the cost is not too great), but after giving the implementation some thought I think it's out-of-scope for this changeset. It's the job of the Noder to average Z values but the noders used in CoverageUnion deliberately avoid the intersection calculations that would produce averages. You can see the same winner-take-all behavior in the polygon CoverageUnion, which is unaffected by this PR:

geosop -a "GEOMETRYCOLLECTION (POLYGON Z ((0 0 0, 1 0 1, 0 1 2, 0 0 0)), POLYGON Z ((1 0 100, 1 1 200, 0 1 300, 1 0 100)))" coverageUnionNG
POLYGON Z ((1 0 1, 0 0 0, 0 1 2, 1 1 200, 1 0 100))

[pramsey]

Can we get a NEWS entry for this feature that notes changes in behaviour for downstream projects (what previously returned a MULTILINESTRING may now return a LINESTRING)?

[dbaston]

It looks like shapely also relies on the current behavior of GEOSDifference to split a line:

https://github.com/shapely/shapely/blob/e10c6da077744005290f77eaa725777d1c631f66/shapely/ops.py#L362

[pramsey]

I have pushed a change to PostGIS to use GEOSSplit over GEOSDifference in the ST_Split function, from GEOS 3.15 and on.