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/lib/i18n/en.ts

@@ -44,6 +44,7 @@ export const en: Catalog = {
     setup: "Setup",
     devices: "Devices",
     tariff: "Tariff",
+    tariffLab: "Tariff Lab",
     subscriptions: "Subscriptions",
     site: "Site",
     users: "Users",
@@ -330,6 +331,36 @@ export const en: Catalog = {
     relayLabel: "Relay {{relay}} ({{direction}})",
     noRelaysConfigured: "This controller has no relays configured.",
   },
+  lab: {
+    title: "Tariff Lab",
+    intro:
+      "Test rates in time (day/night windows, daily caps, overstay) in seconds, with no waiting. Pricing uses the same logic as the booth; nothing is written to the ledger.",
+    loadTicket: "Load from a real ticket",
+    loadTicketPh: "Ticket number / identity",
+    load: "Load",
+    loaded: "Loaded session {{id}}",
+    tariffVersion: "Tariff version",
+    activeVersion: "Active version (current)",
+    entered: "Entered",
+    asOf: "As of (now/exit)",
+    now: "Now",
+    category: "Category",
+    categoryPh: "e.g. bus (blank = car)",
+    payment: "Payment",
+    paid: "paid",
+    graceMin: "grace (min)",
+    price: "Compute price",
+    pricing: "Pricing…",
+    outcome: "Outcome",
+    amountDue: "Amount due",
+    billedPeriod: "Billed period",
+    overstay: "overstay",
+    settled: "settled",
+    periodStart: "Period start",
+    graceExpires: "Grace expires",
+    curve: "Duration curve",
+    curveHint: "Fee from entry at several durations — see where the daily cap flattens or windows shift.",
+  },
   subs: {
     title: "Subscriptions",
     unnamed: "(unnamed)",