chore(miles): remove dead assert_partial_overlap_standalone_safe

[zhenyulincs]

What

Remove the dead function assert_partial_overlap_standalone_safe from miles/utils/rlix_validation.py and its __all__ export entry.

Why

It was already a deprecated no-op (del args; return None), kept — per its own docstring — "for callsite stability." But there are no callsites: an exhaustive search (.py, tests, plans/, docs, dynamic getattr) found the name only in its own definition and the __all__ list.

Its docstring also documents why the guard was neutered in the first place: the standalone create_placement_groups path offsets the rollout pool by actor_num_nodes * actor_num_gpus_per_node, so zero-based train ⊂ infer is also true for valid disjoint standalone configs (e.g. 4 actor GPUs + --rollout-num-gpus 8). Auto-classifying intent from args is unsafe; the RLIX_CONTROL_PLANE env-var is the only reliable standalone signal, and RLix partial-overlap correctness is enforced by assert_rlix_topology (C1). So the function had no remaining purpose.

Safety

• Zero references repo-wide after removal (verified).
• Both the function body and its __all__ entry are removed together, so from rlix_validation import * does not break with a dangling name.
• ast.parse clean; every remaining __all__ entry resolves to a real definition.

Test plan

• Exhaustive grep: no callers, no dynamic references, no test/doc references
• ast.parse OK; all __all__ names resolve