feat(tariff): stepped ("up-to") pricing mode — total-by-duration

Owners often state rates as a total-by-duration matrix (0-1h=200, 0-3h=500,
0-6h=800, 0-9h=900, 0-12h=1000) that the marginal hourly ladder can't express
(the ladder sums per-increment rates; this is cumulative totals at thresholds).
Add STEPPED as a third pricing mode alongside the ladder and flat.

- @parking/shared: TariffStep {uptoMin, totalMinor} + a `steps[]` field on V1
  structures and V2 cards (mutually exclusive with blocks/flatMinor). steppedFee():
  smallest tier with uptoMin >= duration wins (INCLUSIVE boundary), the top tier
  repeats as a per-day cap; wired into computeFeeV1 + computeFeeV2 (V2 default card
  only — a whole-stay total can't be sliced per-increment by a windowed card).
  Validation: ascending uptoMin, non-negative totals, no daily-cap-with-steps,
  steps-only-on-default. priceSession/quote/booth/Lab price it via the shared core.
- Composer UI: a "By duration (up-to)" mode with an up-to/total table (base card
  only). i18n modeStepped/steppedHint/stepUpTo/stepTotal/addStep (sq+en).
- 8 new unit tests incl. the exact owner matrix, multi-day repeat, overstay, and
  validation (53 pass). Verified end-to-end via the UI: authored + published the
  matrix, Tariff Lab prices it exactly (3h->500, 6h->800, 12h->1000, 2d->2000).

Wiki: tariff (three pricing modes + stepped semantics), log.

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

apps/web/src/api.ts

@@ -359,6 +359,8 @@ export interface TariffStructureV1 {
   gracePeriodEntryMin: number;
   incrementMin: number;
   blocks: TariffBlock[];
+  /** STEPPED ("up-to") total-by-duration table; when non-empty it replaces `blocks`. */
+  steps?: TariffStep[];
   dailyCapMinor: number | null;
   lostTicketMinor: number;
   gracePeriodExitMin: number;
@@ -378,8 +380,17 @@ export interface TariffCard {
   window?: TariffWindow;
   flatMinor?: number;
   blocks?: TariffBlock[];
+  /** STEPPED ("up-to") table (defaultCard only); mutually exclusive with flat/blocks. */
+  steps?: TariffStep[];
   dailyCapMinor?: number | null;
 }
+/** One row of a STEPPED ("up-to") tariff: a TOTAL price for a stay up to and including
+ *  `uptoMin` minutes (cumulative, not marginal). Mirrors @parking/shared TariffStep. */
+export interface TariffStep {
+  uptoMin: number;
+  totalMinor: number;
+}
+
 export interface TariffStructureV2 {
   version: 2;
   tz: string;