[lachlangrose]

Draft, stacked. This branch is built on #367, #366, #364 and #363 (it contains their commits), so its diff against master shows all of them until they merge. The change in this PR is the last commit, refactor: PropertyGrid takes its geometry from StructuredGrid3DGeometry and adds cell lookup. Merge order: #363, #366, #364, #367, then this.

Step 3 of consolidating the structured-grid geometry code: the results container reuses the shared geometry class instead of carrying its own copy.

Change

PropertyGrid (the results container renamed in #363) re-implemented maximum, nodes and cell_centres with its own linspace / meshgrid code. It now builds a StructuredGrid3DGeometry from its origin, step_vector and nsteps (the number of nodes) and delegates to it. The dataclass fields are unchanged, so constructor calls, pickles and to_dict are unaffected. The geometry is rebuilt on access rather than stored, so it can never go out of sync with the fields.

New, all from the shared geometry:

• geometry, nsteps_cells, n_nodes, n_cells
• inside(points)
• cell_index_at(points): flat index of the cell each point falls in, in the same order as cell_properties and cell_centres (x fastest); -1 for points outside; points on the maximum boundary belong to the last cell
• cell_value_at(name, points): the cell property in that cell, NaN outside; works for scalar and vector cell data

Points are in the grid's own frame. PropertyGrid has no rotation, and neither lookup applies one.

Tests

packages/loop_common/tests/test_property_grid.py (30 tests, three grids: integer unit grid, offset anisotropic, minimal):

• nodes and cell_centres equal independent meshgrid constructions (so delegating did not change them)

• every cell centre is in its own cell; cell_value_at returns the right values for random points; boundary and outside points; vector-valued cell data

• Mutation check: indexing cell data with node strides instead of cell strides fails 8 of the 30

• packages/loop_common/tests: 213 passed

• packages/loop_interpolation/tests: 606 passed, 46 skipped, 2 xfailed

• tests: 737 passed, 6 skipped

• ruff: no new findings

Not in this PR

• BoundingBox.structured_grid() still returns the interpolation support, not a PropertyGrid.
• Interpolating a node property at a point (trilinear) is not added: it needs the local-coordinate helper, which still lives in the support class.
• The 2D equivalents.

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