Arch: Simplify lb_runner config env fallbacks

[miciav]

Problem

LokiConfig and _apply_env_fallbacks are high-complexity configuration logic blocks.

Evidence

• arch_report/by_target/lb_runner/xenon.txt:1

Plan

1. Extract env fallback logic into small helper functions.
2. Add unit tests for env precedence and defaults.
3. Keep config classes thin and declarative.

Validation

• uv run pytest tests/unit/lb_runner/test_*config*.py
• ./scripts/arch_audit.sh lb_runner