The root cause was PyViewport3DType being declared `static` in
Viewport3D.h, creating per-translation-unit copies. Entity3D.cpp's
copy was never passed through PyType_Ready, causing segfaults when
tp_alloc was called.
Changed `static` to `inline` (matching PyEntity3DType and
PyModel3DType patterns), and implemented get_viewport using the
standard type->tp_alloc pattern.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
