Report the real cause when a vertical plane is queried without depth

[lispandfound]

Fixes #85

Problem

coordinate_length was forced to 3 whenever dip == 90, regardless of the input's dimensionality:

# before
coordinate_length = (
    3 if global_coordinates.shape[-1] == 3 or self.dip == 90 else 2
)

A 2D (lat, lon) query against a vertical plane then sliced 3 components from self.bounds but only 2 from the input, producing ValueError: operands could not be broadcast together with shapes (2,) (3,). Fault.wgs_depth_coordinates_to_fault_coordinates caught that with a bare except ValueError and re-raised "Given coordinates are not on fault." — for a point that is on the fault.

A plane dipping 84 degrees handled the identical query fine, so only the exactly-vertical case failed, and dip == 90 is common in NZ fault models.

What this does not do

The obvious one-line fix — size from the input shape alone — is not enough, and I did not do only that. A vertical plane projects onto a line in plan view, so a (lat, lon) pair maps to every depth on the plane: the dip coordinate is genuinely undetermined. With the slicing merely fixed, the 2x2 system is rank-deficient and lstsq returns a least-norm solution, i.e. a fabricated dip coordinate of roughly 0 that passes the [0, 1] bounds check. That trades a confusing error for a silently wrong answer, which is worse.

So this PR keeps the operation an error, but an honest one. What the correct answer should be — reject, or return a documented convention such as the top edge — is a design question I have left on the issue rather than deciding here.

Changes

1. coordinate_length is derived from the input shape alone.
2. A vertical plane queried without depth raises a specific, descriptive ValueError naming the real cause.
3. New CoordinatesNotOnPlaneError(ValueError) for the genuine off-plane miss. Fault's per-plane loop now catches only that, so the undetermined-dip error surfaces to the caller instead of being reported as a geometric miss. It subclasses ValueError, so any external caller doing except ValueError is unaffected.
4. Docstring Raises section updated; the "not passing depth is supported" note now records that vertical planes are the exception.

Behaviour

query	before	after
vertical plane, 3D	[0.5, 0.5]	[0.5, 0.5]
vertical plane, 2D	broadcast shapes (2,) (3,)	Depth is required to locate coordinates on a vertical plane...
Fault of that plane, 2D	Given coordinates are not on fault.	Depth is required...
plane dipping 84, 2D	[0.5, 0.5]	[0.5, 0.5]
point genuinely off the fault	Given coordinates are not on fault.	Given coordinates are not on fault.

Verification

• 3 new tests; test_vertical_plane_without_depth_reports_the_real_cause fails without the fix.
• Full suite: 721 passed (this touches core geometry, so I ran everything, not just test_sources.py).
• ruff check, ruff format, numpydoc lint clean.

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