feat: add selectable routing groups and composite billing

Support per-model provider enablement and compact model editing. Capture request-time billing factors, charge customer costs separately, and preserve historical statistics without backfills.

crates/aether-testing/integration/src/bin/usage_settlement_hotspot_baseline.rs

@@ -527,6 +527,7 @@ fn settlement_input(index: usize) -> UsageSettlementInput {
         billing_status: "pending".to_string(),
         total_cost_usd: 0.0,
         actual_total_cost_usd: COST_PER_REQUEST_USD,
+        billing_cost_usd: None,
         finalized_at_unix_secs: Some(now_unix_secs().saturating_add(index as u64)),
     }
 }