Add frequency band (high/medium/low) to target results

[kpb]

Goal

• Make output more user-friendly and simplify prompt formatting.

Scope

• Add a frequencyBand field to TargetRow:
  • "high" / "medium" / "low"
• Compute from RecentFrequency thresholds (pick initial values, e.g.):
  • high >= 0.20
  • medium >= 0.10
  • low < 0.10

Acceptance criteria

• Unit tests cover boundaries
• Prompt can use band without re-deriving thresholds