feat(tariff): Tariff Lab — pure session-pricing simulator

Test rates "in time" (overnight windows, daily caps, overstay) in seconds against
any tariff version, instead of waiting hours/days. No real ledger writes.

- Extract priceSession() into @parking/shared: the grace/overstay wrapper over
  computeFee (unpaid -> entry..now; within-grace -> settled 0; grace-expired ->
  overstay, a fresh period from grace-expiry). PayStation.quote() now calls it so
  the booth and the lab can never diverge.
- API (tariffs.ts, tariff:read, read-only): POST /api/tariff/simulate prices a
  hypothetical session (active/any version/inline structure) and returns the
  priceSession outcome + a 30m..3d duration curve (see where the daily cap flattens);
  GET /api/tariff/simulate/session/:identity prefills from a real ledger session.
- UI TariffLab.tsx at Setup -> "Tariff Lab": version picker, entry/asOf times,
  optional payment+grace, category, and load-a-real-ticket. Admin-gated, available
  on-site (useful to quote a dispute).
- 4 new priceSession unit tests incl. the ticket-1245791632490 overstay-not-zero
  regression (40 pass). i18n lab.* + nav.tariffLab (sq+en). Verified live via the UI.

Wiki: tariff (priceSession + Tariff Lab as-built), log.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/web/src/api.ts

@@ -425,6 +425,54 @@ export function publishTariffVersion(body: {
   return apiFetch("/api/tariff/versions", { method: "POST", body: JSON.stringify(body) });
 }
 
+// --- Tariff Lab (simulator) -----------------------------------------------
+
+export interface SimPayment {
+  paidAt: string;
+  graceExitMin: number | null;
+}
+export interface SimSessionPricing {
+  periodStart: string;
+  amountMinor: number;
+  overstay: boolean;
+  withinGrace: boolean;
+  graceExpiresAt: string | null;
+}
+export interface SimulateResult {
+  currency: string | null;
+  pricing: SimSessionPricing;
+  curve: { minutes: number; amountMinor: number }[];
+  gracePeriodExitMin: number;
+}
+export interface SimulateBody {
+  enteredAt: string;
+  asOf: string;
+  payments?: SimPayment[];
+  category?: string;
+  tariffVersionId?: string;
+  structure?: TariffStructure;
+  currency?: string;
+}
+
+/** Price a hypothetical session — pure, no ledger write. See Tariff Lab. */
+export function simulateTariff(body: SimulateBody): Promise<SimulateResult> {
+  return apiFetch("/api/tariff/simulate", { method: "POST", body: JSON.stringify(body) });
+}
+
+export interface SimSessionLoad {
+  identity: string;
+  enteredAt: string;
+  exitedAt: string | null;
+  payments: SimPayment[];
+  category: string | null;
+  tariffVersionId: string | null;
+}
+
+/** Prefill the lab from a real ledger session. */
+export function loadSimSession(identity: string): Promise<SimSessionLoad> {
+  return apiFetch(`/api/tariff/simulate/session/${encodeURIComponent(identity)}`);
+}
+
 // --- Subscriptions --------------------------------------------------------
 
 export interface SubscriptionCredential {