server: pay station + fee calc — full transient loop now passes

computeFee() in @parking/shared: pure integer fee over a TariffStructure
(stepped blocks, rolling-24h cap). Two edges fixed under test: grace uses RAW
duration (not rounded-up minutes), and the block ladder resets each 24h day.

PayStation + routes (GET /api/pay/quote, POST /api/pay): look up the open
session, resolve the active tariff version (latest effectiveFrom <= entry),
computeFee, append a signed payment event (amount/currency/tender/
tariffVersionId/graceExitMin). overrideMinor handles lost-ticket/dispute. PCI
stays out of the app: tender only records cash/card.

Verified end to end: entry -> quote (300 for 90min) -> pay -> exit opens and
closes the session, verifyChain ok.

wiki/concepts/tariff.md

@@ -87,6 +87,15 @@ Deterministic, side-effect-free, unit-testable; the daily cap is applied **per r
 overnight stay doesn't hit the cap twice). Rounding and segment edges are part of the settled spec
 because the chain + reconciliation depend on the result being reproducible.
 
+**Settled edges (2026-06-15, with tests):**
+- **Grace uses RAW duration** — a stay within `gracePeriodEntryMin` is free even though the
+  increment would round it up (else rounding defeats the grace window).
+- **The block ladder RESETS each rolling-24h day** — day 2 starts at the first block again (a 25h
+  stay = day-1 capped + day-2 first-hour rate), so the "daily" rate truly resets daily.
+
+**As-built:** `computeFee(enteredAt, asOf, structure)` in `packages/shared` (pure). Unit-tested
+across grace, block steps, daily cap, and multi-day reset.
+
 ## The pay-on-foot consequence
 
 Because payment is decoupled from exit ([[parking-session]] lifecycle), the tariff has **two