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/lib/i18n/en.ts

@@ -237,6 +237,12 @@ export const en: Catalog = {
     defaultCardHint: "The base rate applied when no time/seasonal tier matches. This alone is enough for most car parks.",
     modeLadder: "Hourly ladder",
     modeFlat: "Flat price",
+    modeStepped: "By duration (up-to)",
+    steppedHint:
+      "Set the TOTAL price for a stay up to a given time (e.g. up to 3h = 500). The first row whose limit ≥ the duration wins (the limit is inclusive). The last row's total repeats as a per-day price for longer stays.",
+    stepUpTo: "Up to",
+    stepTotal: "Total price",
+    addStep: "+ Add row",
     tiersAdvanced: "Advanced: time & seasonal tiers",
     tiersHint: "Optional. Add tiers that apply only at certain hours/days/dates or for a category (e.g. happy hour, night rate, weekend, bus). With no tiers, just the base rate is published.",
     tierName: "Name",