Fix translucent rendering updates for STEP-backed glTF models

[seveibar]

Motivation

• STEP files are converted to GLB and cached, but toggling the "show as translucent model" flag did not always update already-loaded glTF materials.
• Material transparency updates were applied only during model load, causing the UI toggle to require a reload to take effect for STEP-backed models.

Description

• Load the glTF once and keep loading tied only to gltfUrl so the model is not reloaded when translucency changes.
• Move material translucency updates into a dedicated effect that runs when the loaded model or isTranslucent changes, and apply transparent, opacity, depthWrite, and needsUpdate to each material.
• Support both single-material and multi-material meshes during translucency updates so all mesh materials reflect the current isTranslucent state.

Testing

• bunx tsc --noEmit completed successfully and reported no type errors.
• bun test tests executed 12 tests across 7 files with 10 passing and 2 failing tests in tests/preprocess-circuit-json.test.ts, which are unrelated to the glTF translucency changes.
• bun run format ran and formatted files successfully.

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Codex Task

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Project	Deployment	Actions	Updated (UTC)
3d-viewer	Ready	Preview, Comment	Feb 14, 2026 5:12am