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/router.tsx

@@ -21,6 +21,7 @@ import { StatusDot } from "./ui/StatusDot.js";
 import { BoothScreen } from "./BoothScreen.js";
 import { SetupWizard } from "./SetupWizard.js";
 import { TariffComposer } from "./TariffComposer.js";
+import { TariffLab } from "./TariffLab.js";
 import { SubscriptionManager } from "./SubscriptionManager.js";
 import { ShiftControl } from "./ShiftControl.js";
 import { SiteSettings } from "./SiteSettings.js";
@@ -80,6 +81,7 @@ function SetupLayout() {
       <nav className="mb-4 flex flex-wrap items-center gap-1 border-b border-term-border">
         {show("site:update") && <SetupTab to="/setup" label={t("nav.devices")} exact />}
         {show("tariff:read") && <SetupTab to="/setup/tariff" label={t("nav.tariff")} />}
+        {show("tariff:read") && <SetupTab to="/setup/tariff-lab" label={t("nav.tariffLab")} />}
         {show("subscription:read") && <SetupTab to="/setup/subscriptions" label={t("nav.subscriptions")} />}
         {show("site:read") && <SetupTab to="/setup/site" label={t("nav.site")} />}
         {show("user:read") && <SetupTab to="/setup/users" label={t("nav.users")} />}
@@ -395,6 +397,12 @@ const tariffRoute = createRoute({
   beforeLoad: ({ context }) => requirePerm("tariff:read")(context),
   component: () => <TariffComposer />,
 });
+const tariffLabRoute = createRoute({
+  getParentRoute: () => setupRoute,
+  path: "tariff-lab",
+  beforeLoad: ({ context }) => requirePerm("tariff:read")(context),
+  component: () => <TariffLab />,
+});
 const subscriptionsRoute = createRoute({
   getParentRoute: () => setupRoute,
   path: "subscriptions",
@@ -456,6 +464,7 @@ const routeTree = rootRoute.addChildren([
   setupRoute.addChildren([
     setupDevicesRoute,
     tariffRoute,
+    tariffLabRoute,
     subscriptionsRoute,
     siteRoute,
     usersRoute,