[xixihaha-ha]

Plane3.intersection raises a matrix multiplication error even for three planes with a unique intersection: np.linalg.det(A) is a scalar. Solve A @ point = -b instead, following the documented plane equation n @ point + d = 0.

Reproduction on the base revision:

import numpy as np
from spatialmath import Plane3

planes = [Plane3([1, 0, 0, -1]), Plane3([0, 1, 0, -2]), Plane3([0, 0, 1, -3])]
np.testing.assert_allclose(Plane3.intersection(*planes), [1, 2, 3])

Validation on Windows / Python 3.11:

• All 14 added regression cases fail on the original implementation.
• pytest tests/test_geom3d.py -q: 56 passed.
• pytest -q: 487 passed, 3 skipped; 63 subtests passed.
• Black 23.10.0 checks for both changed files and git diff --check passed.

Tests cover known intersections, oblique normals, signed coefficient scaling, plane order, equation residuals, and singular systems. No dependencies or public signatures change.

The human requested a robotics-related bug fix and authorized submission. Codex diagnosed the problem, implemented the one-line fix, and wrote and ran the tests.

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✅ All modified and coverable lines are covered by tests.

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