refactor(rlix-examples): extract shared driver helpers into _common

[TianyeGGBond]

Context

examples/rlix/run_miles_rlix.py (single) and run_miles_dual.py (dual)
carried three near-verbatim copies of the same logic:

• the RLIX_CONTROL_PLANE=rlix env-var guard (must run before heavy imports),
• the MilesPipelineConfig dataclass (defined as a local class in each
  main(); the dual one had an extra cluster_device_mappings field),
• the Ray runtime_env env_vars construction (identity vars + PYTHONPATH +
  the forwarded smoke-only escape hatches).

Two drifting copies are easy to let diverge.

Change

New examples/rlix/_common.py (stdlib-only, so it is safe to import before
the per-actor CUDA_VISIBLE_DEVICES guard fires):

• require_rlix_control_plane(script_path, module_path) — the guard
• MilesPipelineConfig — one dataclass; cluster_device_mappings defaults
  to {}, so the single driver (which never set it) is unaffected
• build_pipeline_runtime_env(pipeline_id, namespace, *, extra=None) — the
  env_vars dict; the dual driver passes MILES_ROLLOUT_BASE_PORT via extra=

Both drivers now import these instead of redefining them. No behavior change.

Stacking

Stacked on #18 (partial-env fail-fast). Until #18 merges into
zhenyu/m11-mvp-test, this PR's Files-changed view also shows #18's commit;
review the top commit here. Merge after #18.

🤖 Generated with Claude Code

[TianyeGGBond]

Updated (force-push) to fix a blocker: the relative from ._common import ...
broke direct-script invocation (python examples/rlix/run_miles_*.py), which
the rlix smoke scripts use — ImportError: attempted relative import with no known parent package.

Both driver _common imports now use a try/except fallback (from ._common
→ from _common), so both python -m examples.rlix.run_miles_* and
python examples/rlix/run_miles_*.py work. Verified both invocation styles
resolve _common (heavy imports still deferred past the env guard).