Dispose temporary STEP export resources

[zergzorg]

/claim #93

Summary

• dispose the temporary STEP conversion group after GLTFExporter succeeds or fails
• dispose mesh geometries, materials, and common/newer material texture slots via a shared helper
• add coverage for single-material, multi-material, and MeshPhysicalMaterial texture disposal paths

Effect

The STEP conversion path builds an intermediate THREE.Group only to export a GLB buffer. Before this change, those temporary BufferGeometry, Material, and Texture objects stayed undisposed after export, so repeated STEP loads could keep GPU-side resources around longer than needed. The GLB cache and rendered model behavior are unchanged.

Scope and non-overlap

This PR is a cleanup/resource-disposal slice for temporary STEP export resources. It complements cache-focused work on issue #93: cache changes reduce repeated loading, while this PR makes sure the intermediate Three.js resources created during STEP export are released whether export succeeds or fails.

Validation

• bun test tests/dispose-object3d-resources.test.ts
• bun run build
• bunx biome format src/utils/dispose-object3d-resources.ts src/three-components/StepModel.tsx tests/dispose-object3d-resources.test.ts
• git diff --check

Issue: #93

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Project	Deployment	Actions	Updated (UTC)
3d-viewer	Ready	Preview, Comment	May 24, 2026 3:15pm

[zergzorg]

Follow-up hardening pushed in 20fd21c:

• expanded the shared disposal helper to cover newer Three.js material texture slots such as clearcoat/transmission/specular/sheen maps
• added a focused regression test for MeshPhysicalMaterial extension texture disposal

Validation:

• bun test tests/dispose-object3d-resources.test.ts
• bunx biome format src/utils/dispose-object3d-resources.ts tests/dispose-object3d-resources.test.ts
• bunx tsc --noEmit
• GitHub format-check, test, and type-check are passing on the new head

[zergzorg]

Follow-up hardening pushed in 625cc3f:

• kept shared envMap textures out of the temporary STEP resource disposer, since environment maps are commonly shared scene assets rather than resources owned by the converted mesh
• added regression coverage proving the helper still disposes owned geometry/material texture slots while leaving envMap intact

Validation:

• bun test tests/dispose-object3d-resources.test.ts
• bunx biome format --write src/utils/dispose-object3d-resources.ts tests/dispose-object3d-resources.test.ts
• bunx biome check src/utils/dispose-object3d-resources.ts tests/dispose-object3d-resources.test.ts
• bunx tsc --noEmit
• bun run build
• git diff --check