Arch: Refactor BenchmarkController._prepare_run_state

[miciav]

Problem

_prepare_run_state is a complexity hotspot with many responsibilities.

Evidence

• arch_report/xenon.txt:4
• arch_report/lizard.txt:91
• lb_controller/engine/controller.py:147

Plan

1. Extract inventory + run id resolution into helpers.
2. Extract run directory prep into a small service.
3. Extract extravars assembly into a dedicated builder.
4. Add unit tests for each builder.

Validation

• uv run pytest tests/unit/lb_controller/test_*controller*.py
• ./scripts/arch_audit.sh lb_controller