Runs the price rule engine over a proposed set of line items without persisting. Used by the invoice grid to show rule discounts (BOGO, spend thresholds, etc.) live as the user edits, while keeping the server as the single source of truth for the math — no client-side evaluator to drift.

Estimate's client, used to resolve customer-tag rule gating in preview. Optional — when absent, client-tag-gated rules are simply not previewed.

Invoice's patient — their tags also satisfy tag gating (ITE-660: plan-member pricing is a patient-assigned tag). Optional, same preview-only semantics.