Arch: Refactor RichDashboard log rollups

[miciav]

Problem

RichDashboard mixes UI rendering with complex log parsing/rollup logic.

Evidence

• arch_report/by_target/lb_ui/hotspots.txt:1
• arch_report/by_target/lb_ui/xenon.txt:8
• lb_ui/tui/system/components/dashboard.py:234

Plan

1. Extract parsing/rollup logic into a dedicated helper class.
2. Keep UI rendering methods in RichDashboard only.
3. Add unit tests for rollup parsing and summary generation.

Validation

• uv run pytest tests/unit/lb_ui/test_*dashboard*.py
• ./scripts/arch_audit.sh lb_ui