[willend]

Fixes #398.

The MCDISPLAY primitives drew some shapes differently from the McCode runtime (mccode-r.c) and from McCode's own mcdisplay tools (mccode-dev/McCode#2844, mccode-dev/McCode#2846). This PR brings to_polylines, to_mesh and the brep renderer in line with them.

Box (box(x,y,z, xw,yh,zd, thickness, nx,ny,nz))

• thickness > 0 now draws a closed xw-2t × yh-2t × zd-2t cavity, as mcdis_box does. Before, it drew xw-t × …, from half-sizes d - t/2. The cavity is drawn only when all three sizes are positive.
• The normal is applied everywhere: local y is rotated onto n by the smallest rotation (a half turn about z for n = -y), as mcdis_rotated_box does. Before, to_polylines and to_mesh ignored n, and brep rotated local z onto n. In brep that also scrambled the axes of a box with the default normal: Box(0,0,0, 0.1,0.2,0.3) came out 0.2 × 0.3 × 0.1.
• to_mesh and brep now include the cavity. The cavity's mesh triangles are wound in reverse.

Cylinder

• thickness > 0 adds the closed r-t, h-2t cavity, as mcdis_cylinder does. This applies in to_polylines, to_mesh and brep.

Annulus

• The 5th argument is the ring width, as McStas and McXtrace Annulus.comp pass it (mcdis_annulus(0,0,0, radius, thickness, 0,1,0)). The inner radius is now max(0, r_outer - width), so Annulus(radius=0.06, thickness=0.02) gives a ring over [0.04, 0.06] instead of [0.02, 0.06].
• The dataclass field is renamed r_inner → thickness, to match its meaning and the McCode argument name. This is an API change for anyone who builds Annulus(..., r_inner=...) by keyword (breaking-change label suggested). Positional construction and parsing are unchanged.

Tests

• New TestHollowShapes classes in tests/test_display.py and tests/test_display_brep.py cover box rotation (+x, and -y as a half turn), the cavity sizes, the solid fallback when the walls are too thick, the cylinder cavity, the brep volumes, and the annulus radii.
• I ran each case against the code on main: the old code gives a 0.09×0.19×0.29 cavity, an unrotated box, no cylinder cavity, an annulus inner radius of 0.02, and a brep default box of 0.2×0.3×0.1.
• python -m pytest tests (macOS arm64, [test,brep] extras plus matplotlib, McCode latest): 1321 passed, 8 skipped, 0 failed. I did not run it against the v3.8.7 pin, since the change does not touch compiling or running instruments. Not run on Linux or Windows.

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