perf: cache render graph imports for static scene resources

[L-Sun]

Summary

• keep imported render-graph resource nodes alive across frame resets and reuse freed node slots for transient frame data
• cache scene mesh, material, and texture imports in ForwardRenderer so static scene resources are not re-imported every frame
• invalidate the persistent caches cleanly when the active scene changes while keeping per-frame constant buffers transient

Testing

• xmake build editor