rbio: add @requires decorator + mtime-gated common sdist rebuild

[davidsmejia]

Issue Number

#3572

Purpose/Implementation Notes

• bin/lib/_requirements.py (new) — @requires(env=, env_optional=, tools=)
  decorator + epilog_from() helper. Commands declare their env vars and
  external tool deps; future debug:env / debug:deps will read this
  registry to render per-command and aggregate views.
• bin/lib/common.py — factor the inline sdist rebuild out of cmd_common_update
  into rebuild_common_sdist(), which now:
  • uses an mtime gate to skip the rebuild when no common source file
    is newer than the existing sdist (preserves docker COPY layer cache
    for downstream worker/foreman/api images)
  • prefers dr_env/bin/python3 when the venv is present (system python3
    lacks setuptools on PEP-668 distros)
    Surfaces the helper as a standalone command, rbio common:build-sdist.
• bin/lib/build.py — invoke rebuild_common_sdist() before bake so
  downstream targets pick up the latest common package. No-op when source
  is unchanged (gate above).

Methods

N/A

Types of changes

• New feature (non-breaking change which adds functionality)

Functional tests

rbio functional tests only

Checklist

• Lint and unit tests pass locally with my changes

Screenshots

N/A