Fix inverted coordinate conversion in test_top_bottom_point

[lispandfound]

Fixes #95

Problem

test_top_bottom_point ran the coordinate conversion backwards:

# before
point = Point(
    coordinates.nztm_to_wgs_depth(np.array([-43.0, 172.0, 1000.0])),
    ...
)

Point.bounds is documented as NZTM (sources.py:37-38) and Point.from_lat_lon_depth builds it with coordinates.wgs_depth_to_nztm(...). The literal here is already a WGS lat/lon, so applying nztm_to_wgs_depth to it produces neither valid NZTM nor the intended location:

intended location           : [ -43.  172. 1000.]
old form, point.coordinates : [ -75.81  82.93 1000.]     <- not in New Zealand
fixed,    point.coordinates : [ -43.  172. 1000.]

Why it passed anyway

The test only asserts on point.top_m and point.bottom_m, which depend solely on the depth component — and nztm_to_wgs_depth passes depth through untouched. top_m/bottom_m are byte-identical either way, so the assertions were satisfied no matter how wrong the horizontal position was. The test did not exercise what it appeared to, and the construction is the kind of thing that gets copied into a test that does use the horizontal position.

Changes

• Use coordinates.wgs_depth_to_nztm(...).
• Add assert np.allclose(point.coordinates, point_coordinates) so the horizontal components are actually pinned down.

Verification

• pytest tests/test_sources.py — 64 passed.
• The new round-trip assertion fails with the old construction, confirming it is not vacuous.
• ruff check, ruff format clean.

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