WebGPURenderer: Fix getArrayBufferAsync in WebGPUBackend

[RenaudRohlinger]

Description

Fix GPU buffer readback by correctly sequencing map/unmap operations. It was necessary to create a detached copy of mapped data before unmapping to ensure buffer data remains accessible after the GPU operation completes.
There is no need to update the WebGLBackend as it was already properly returning a detached typed array.

I also noticed the audio example wasn't working anymore in production (related #29972), so I fixed it.

This contribution is funded by Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.24 79.02	339.24 79.02	+0 B +0 B
WebGPU	486.17 134.89	486.27 134.93	+103 B +36 B
WebGPU Nodes	485.63 134.79	485.74 134.83	+103 B +34 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.13 112.08	465.13 112.08	+0 B +0 B
WebGPU	555.32 150.32	555.42 150.35	+103 B +34 B
WebGPU Nodes	511.24 140.09	511.34 140.12	+103 B +35 B