ICYLAB Billing Finance Read Isolation v6.12

Changes:
- BillingReceivablesService list/detail no longer auto-link, materialize or recalculate on read.
- BillingPayablesService list/KPI/detail no longer resolve, materialize or recalculate on read.
- BillingSupplierAgingService summary/detail no longer trigger mass maintenance.
- Payable overdue status is projected in queries/memory without persistence.
- Explicit financial actions remain protected by FINANCE gate and keep targeted writes.
