fix: fall back to base splat runtime during export

[Sportinger]

Summary

• fall back to a base Three.js gaussian splat runtime when precise export cannot allocate the full target runtime
• keep export readiness cached so the failing target build is not retried on every frame
• add regression coverage for the export-readiness fallback path

Verification

• npm run build
• npm run lint
• npm run test

No version bump in this release path per request.

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