Preserve MultiMaterial output in CSG mesh merges

[tracygardner]

Motivation

• Fix a bug where merging/CSG operations always overwrote the merged mesh material with the first input material, collapsing different colored submeshes into a single material.
• Ensure the fallback Mesh.MergeMeshes preserves submesh/MultiMaterial output so material indices produced by CSG/merge are not lost.

Description

• Modify flockCSG.mergeMeshes() to prefer and preserve the mergedMesh.material when CSG or Mesh.MergeMeshes produces a MultiMaterial or valid material output instead of unconditionally replacing it (file changed: api/csg.js).
• Enable the Mesh.MergeMeshes flags that preserve submeshes/MultiMaterial data (subdivideWithSubMeshes and the other relevant boolean) when falling back from CSG.
• Add guarded fallback logic that only replaces missing or default materials with a clone of the reference mesh material, using an isDefaultMaterial helper and preserving backFaceCulling settings.
• Leave existing metadata, normals rebuild, and physics-shape creation flow intact.

Testing

• Ran npm run build, which completed successfully (build produced the dist output) ✅.
• Ran npm run test:api, which failed in this environment because Playwright browser binaries are not installed and must be fetched via npx playwright install (test runner aborted) ⚠️.

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Codex Task