Prefer container infra binaries and prewarm Pyxis images

[nv-roverman]

Summary

• Prefer NATS and etcd binaries available inside the container.
• Preserve support for explicitly mounted infra binaries.
• Prewarm Pyxis images before launching distributed services and workers.

Testing

• uv run pytest tests/test_benchmarks.py

[codecov-commenter]

Codecov Report

❌ Patch coverage is 5.40541% with 35 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (sa-submission-q2-2026@fdb1be7). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/srtctl/cli/setup_head.py	0.00%	21 Missing ⚠️
src/srtctl/cli/do_sweep.py	12.50%	14 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##             sa-submission-q2-2026     #140   +/-   ##
========================================================
  Coverage                         ?   60.63%           
========================================================
  Files                            ?       48           
  Lines                            ?     4174           
  Branches                         ?        0           
========================================================
  Hits                             ?     2531           
  Misses                           ?     1643           
  Partials                         ?        0           

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