[wgpu] add convience functions for deferring mapping/callbacks

[cwfitzgerald]

Connections

None

Description

This adds various functions for making dealing with mapping and other callbacks easier. Often you are working with a command buffer then want to map a buffer or do something when that command buffer is submitted. Normally you need to build some kind of infrastructure for this (I have many times) but this can be really easily be done by wgpu itself for very little cost.

Testing

Told GPT-5 to put together some tests as an experiment. I think it did okay.

Squash or Rebase?

Rebase me pls

Checklist

• Baseline implementation
• Improved code documentation
• Validation tests
• Better verbiage
• Changelog

[Wumpf]

neat!
I think it's rather unfortunate to have this kind of extension in wgpu proper rather than an extension to it. Makes me wonder whether those deferred commands could be more generic and then we expose the concrete buffer extension as a utility.... but I don't want to throw a wrench into a perfectly good feature that is really hard to get into the ecosystem if we don't put it in here. So overall slight scope creep concerns, but let's ship this anyways!