Defer WebGPU buffer destruction until submit

[slimbuck]

Summary

• Defers GPUBuffer.destroy() in WebgpuBuffer.destroy() until pending command buffers have been submitted.
• Matches the existing deferred-destroy behavior used for WebGPU textures.

Details

A GPUBuffer can still be referenced by recorded command buffers through bind groups even after the engine-side buffer wrapper is destroyed. Destroying the underlying WebGPU buffer immediately can invalidate a later queue.submit() with validation errors such as “buffer used in submit while destroyed”.

This change routes buffer destruction through device.deferDestroy(...), so the underlying GPUBuffer is destroyed only after pending command buffers are submitted.

Test Plan

• Verified WebGPU picking no longer reports “Buffer used in submit while destroyed” during gsplat interaction.
• Existing WebGPU texture destruction already follows this deferred lifetime pattern.

[Copilot]

Pull request overview

Defers destruction of underlying WebGPU buffers so they aren’t destroyed while still referenced by already-recorded command buffers (e.g., via bind groups), matching the existing deferred-destroy pattern used for WebGPU textures.

Changes:

• Route WebgpuBuffer.destroy() through device.deferDestroy(...) instead of calling GPUBuffer.destroy() immediately.
• Add clarifying inline comments explaining why destruction must be deferred.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.