Centralize safe runtime port allocation

[weireweire]

Summary:

• Revert per-job SGLang port jitter in favor of centralized low, non-ephemeral runtime port ranges.
• Add preflight warnings for planned ports inside the kernel ephemeral range.
• Adjust busy planned worker ports before launch and log the old and new values.
• Update backend/frontend/infra defaults and related unit tests.

Validation:

• python3 -m py_compile on changed Python files and targeted tests
• bash -n src/srtctl/benchmarks/scripts/profiling/profile.sh
• git diff --check

Not run:

• pytest/ruff, because uv/pytest/ruff are not installed in this local environment and the system Python is too old for package runtime imports.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.14286% with 10 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@9cd8fe8). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/srtctl/cli/setup_head.py	0.00%	6 Missing ⚠️
src/srtctl/cli/mixins/benchmark_stage.py	33.33%	2 Missing ⚠️
src/srtctl/cli/mixins/worker_stage.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #156   +/-   ##
=======================================
  Coverage        ?   65.04%           
=======================================
  Files           ?       67           
  Lines           ?     8207           
  Branches        ?        0           
=======================================
  Hits            ?     5338           
  Misses          ?     2869           
  Partials        ?        0           

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[weireweire]

tested on mars and sglang and passed.