`Domain.build()` fails after domain addition (`ValueError: data must be 2 dimensions`)

[zasexton]

Description

Calling build() on a composite Domain (created via cube + sphere) raises a ValueError from scipy.spatial.cKDTree indicating the input data is not 2-D. Building each domain individually works; the failure occurs only after combining domains.

Reproduction

from svv.domain.domain import Domain
import pyvista as pv

cube = Domain(pv.Cube())
cube.create()
cube.solve()
cube.build()

sphere = Domain(pv.Sphere())
sphere.create()
sphere.solve()
sphere.build()

dom = cube + sphere
dom.build()              # <-- crashes
# dom.solve(); dom.build() also crashes

Expected behavior

build() should succeed for composite domains created via add, producing a valid function KD-tree (or equivalent structure) for the merged domain.

Additional context

No response

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