refactor: share scene load coordination

[alexchuber]

Summary

• centralize per-scene load coordination in the shared loading module
• keep glTF format detection and dependency interpretation in the glTF input block
• preserve redirect-derived metadata, cancellation, and existing input behavior

Validation

• integrated against current main, including OBJ input support
• pnpm test
• pnpm format:check
• pnpm typecheck
• pnpm build
• git diff --check