Avoid advancing RNG when creating agg devices

[gjhunt]

Summary

This PR replaces RNG-based device naming (sample()) with tempfile()-based names so that creating agg devices no longer advances the global RNG state.

Motivation

Device creation currently mutates .Random.seed, which can be surprising in reproducible workflows. This can also make whitespace semantically meaningful in downstream jupyter contexts, e.g., latest containerized jupyter/datascience-notebook.

Changes

• Use basename(tempfile()) to generate unique device names without touching RNG state
• Add tests asserting that agg_capture() and agg_record() do not advance the global RNG

Notes

• tempfile() does not create files and does not use or modify .Random.seed
• Behavior is unchanged except for the removal of RNG side effects during device creation

[thomasp85]

Thanks!