Three exporters (STL/OBJ/GLTF) crash when traversing a Mesh with missing/disposed/empty geometry (EDITOR-6H/6G/79). `prepareSceneForExport()` clones the scene and removes ONLY meshes whose position attribute is missing or zero-count — collected in a single traverse and removed afterward (no mutation during traversal). Lines, points, groups, cameras, lights, bones and valid skinned meshes are preserved, so OBJ/GLB output is no longer corrupted. STL/OBJ still throw and GLTF still rejects on other errors — no silent no-op.
Reimplemented against main without the original PR's all-non-mesh pruning, mutate-during-traverse loop, and error-swallowing try/catch. Verified: `tsc -p apps/editor` clean, biome clean.
