Dispose MixedStlModel fallback resources

[swhan0329]

/claim #93

Summary

• Track whether MixedStlModel is rendering its locally-owned red fallback mesh or a loaded cached model.
• Dispose the fallback mesh geometry/material when it is replaced or unmounted.
• Reuse already-registered STEP-to-GLB blob URLs before reading the localStorage GLB cache again, avoiding duplicate object URLs for repeated STEP mounts.
• Keep loaded OBJ/WRL model instances untouched so shared/cached loader resources are not accidentally disposed.
• Add focused helper tests for owned-resource disposal and STEP blob URL reuse.

Why

When a board has many duplicate or slow-loading models, MixedStlModel creates fallback meshes while the model loader is pending. The transform graph removes those placeholders when the real model arrives, but the placeholder geometry/material were not disposed. The follow-up also closes a small STEP cache reuse gap where a cached GLB could create another blob URL even though the global conversion registry already had one for the same STEP URL.

Verification

• bun test tests/step-model-cache.test.ts tests/dispose-owned-object-resources.test.ts
• bun run format:check
• bunx tsc --noEmit
• bun run build
• bun run test:node-bundle
• git diff --check

Full test note

bun test previously had 3 failures unrelated to this change in this checkout:

• tests/preprocess-circuit-json.test.ts: expected faux-board z offsets are 0.1 higher than current output.
• tests/outline-bounds.test.ts: expected soldermask color string does not match current rendered SVG color.

AI-assisted with Codex; I reviewed the diff and verification output before opening and updating this PR.

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Project	Deployment	Actions	Updated (UTC)
3d-viewer	Ready	Preview, Comment	May 20, 2026 2:37am

[swhan0329]

Review-ready follow-up: all attached checks are green on this PR (format-check, test, type-check, Vercel, and Vercel Preview Comments), and the patch stays scoped to locally-owned fallback/STEP blob URL cleanup without touching shared OBJ/WRL cached model instances. Happy to adjust if maintainers want this split smaller.