Add Mesh Surface

Class skeleton

class NewShapeMeshSurface(MeshSurface3D):
    """One-line description.

    Longer docstring explaining the geometry and what MPIEMSES primitive
    (if any) this corresponds to.
    """

    def __init__(
        self,
        center: Union[Tuple[float, float], Tuple[float, float, float], np.ndarray],
        axis: AxisSpec,
        # ... geometry params in grid units ...
        *,
        # resolution params, theta_range, parts, flip_normal, ...
    ):
        self.center = _center_to_3vec(center)
        self.axis, self.e1, self.e2 = _orthonormal_frame(axis)
        # validate + stash params
        self.ntheta = _normalize_count(ntheta, name="ntheta", minimum=3)
        # etc.

    def mesh(self) -> Tuple[np.ndarray, np.ndarray]:
        # Build (nv, nu, 3) points or call a helper.
        return _plane_mesh(points, expected_normal=tuple(self.axis), wrap_u=False)

Conventions:

• Every length is in grid units. Unit conversion (use_si) is the boundary layer's concern, not the mesh class.
• Axial range (length vs tmin/tmax) goes through _axial_range(length=..., tmin=..., tmax=...).
• Angular range goes through _resolve_theta_range(theta_range) + _sample_theta(...).
• If your shape has multiple parts (e.g. side + top + bottom), accept a parts= kwarg and normalize with _normalize_selection(value, allowed=self._allowed_parts, name="parts").
• If your shape has a natural outward normal, accept flip_normal=False and, at mesh() time, pass the flipped or unflipped axis to _plane_mesh via expected_normal.
• Validation errors are plain ValueError with a short, specific message.

Wiring checklist

When the class is written:

1. Add it to the __all__ list at the bottom of mesh.py.
2. Add it to the from .mesh import (...) block and the __all__ list in emout/plot/surface_cut/__init__.py.
3. If this mesh is meant to be consumed by data.boundaries[i], also add a matching Boundary subclass via the add-boundary skill and register it in _BOUNDARY_CLASS_MAP.
4. Run run-tests skill or pytest tests/plot/test_surface_cut_mesh.py -q at minimum.

Test pattern

Add a test to tests/plot/test_surface_cut_mesh.py. Look at test_sphere_mesh_surface_points_lie_on_sphere or test_rectangle_mesh_surface_builds_flat_panel as templates. Assertions worth including:

• Shape of V and F (counts depend on ntheta/nradial — compute, do not hardcode).
• Geometric invariants: points lie on the expected surface, extrema match the declared bounds, no degenerate triangles when they shouldn't exist.
• For closed solids, the mesh is watertight-enough for visualization (every edge appears in ≥ 1 triangle — you rarely need to strictly check watertightness for viz meshes).
• If the class supports theta_range, include a half-section test.
• If the class supports flip_normal or face selection, test at least one non-default choice.

Common mistakes

• Forgetting wrap_u=True on cylindrical/annular patches. Missing wrap leaves a visible seam.
• Passing expected_normal in the wrong sign. _orient_faces_to_normal flips winding to match, so if the result renders inside-out, flip the sign you pass.
• Mutating shared numpy arrays. center, e1, e2 are stored on self; if you compute points = center[None, None, :] + ..., do not then write back into center.
• Accepting a length AND tmin/tmax. _axial_range rejects this combination. Let the helper enforce it.
• Hard-coding (0, 0, 1) as the axis. Always go through axis, e1, e2 = _orthonormal_frame(axis).