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

wiki/concepts/tariff.md

@@ -101,7 +101,12 @@ because the chain + reconciliation depend on the result being reproducible.
   cap" model made complete — the same engine, no new axis; the only gap was the unstated tail.
 
 **As-built:** `computeFee(enteredAt, asOf, structure)` in `packages/shared` (pure). Unit-tested
-across grace, block steps, daily cap, and multi-day reset.
+across grace, block steps, daily cap, and multi-day reset. A higher-level **`priceSession(enteredAt,
+asOf, structure, payments[], category?)`** (also pure, shared) wraps `computeFee` with the
+grace/overstay logic — unpaid → entry→now; paid+within-grace → settled (0); paid+grace-expired →
+**overstay**, a fresh period from grace-expiry→now (see [[booth-exit-flow]]). The booth's
+`PayStation.quote()` and the [[#tariff-lab-simulator-as-built-2026-06-20|Tariff Lab]] both call it, so
+live pricing and the simulator can never diverge.
 
 ### Composer (as-built 2026-06-15)
 
@@ -124,6 +129,25 @@ The admin authors the rate card at runtime — no hand-seeding:
 - Ships **blank** — until a version is published, `GET /api/tariff` returns `active: null` and the
   pay station returns `409 no active tariff`. Verified end to end (publish → pay station prices).
 
+### Tariff Lab (simulator, as-built 2026-06-20)
+
+The tariff engine is a **pure function of time**, but you could previously only *exercise* it by
+waiting (the only clock the booth reads is the real wall-clock). The **Tariff Lab** closes that gap:
+price a session at **any** instant against **any** tariff version in seconds.
+
+- **API** (`apps/server/src/routes/tariffs.ts`, `tariff:read` — admins always have it; available
+  on-site too, useful to quote a customer dispute): `POST /api/tariff/simulate` prices a hypothetical
+  session — body `{enteredAt, asOf, payments[], category?, tariffVersionId? | structure?}` — and
+  returns the full `priceSession` outcome plus a **duration curve** (fee from entry at 30m…3d, so you
+  SEE where the daily cap flattens or a window shifts). `GET /api/tariff/simulate/session/:identity`
+  prefills from a **real ledger session** (entry + payments + the version frozen at entry). Both are
+  **read-only — no ledger writes.**
+- **UI** (`apps/web/src/TariffLab.tsx`, Setup → "Tariff Lab"): pick a version (active or any
+  historical), set entry / "as of" times, an optional payment (with its grace), and a category; or
+  "Load" a real ticket to re-evaluate it at any moment. Shows amount due, billed period, overstay/
+  settled state, and the curve. Prices via the same `priceSession` the booth uses (verified: a real
+  overstay ticket reads identically in the lab and the booth). See [[booth-exit-flow]] (overstay).
+
 ## The pay-on-foot consequence
 
 Because payment is decoupled from exit ([[parking-session]] lifecycle), the tariff has **two