Package weight, payload capacity and per-kg payout (ISS-045)

Packages carry a real weightKg (unit weight x count with +/-30% jitter),
plane capacity becomes maxPayloadKg instead of 6 slots, load validation
sums cargo weight against the payload, and the payout gains a per-kg part.
Loadout display shows tonnage; OpenAPI swaps CatalogEntry.capacity for
maxPayloadKg and adds PackageState.weightKg.

lib/deriveEvents.test.ts

@@ -6,7 +6,7 @@ import { diffSnapshots } from './deriveEvents';
 // feature, unit-tested in node like advanceSimClock/fetchState.
 
 const CATALOG = [
-  { model: 'atr72', label: 'ATR 72-600', price: 200_000, maxFuel: 6300, burnPerKm: 1.75, emptyWeightKg: 9300, speedKmh: 510, capacity: 6 },
+  { model: 'atr72', label: 'ATR 72-600', price: 200_000, maxFuel: 6300, burnPerKm: 1.75, emptyWeightKg: 9300, speedKmh: 510, maxPayloadKg: 4800 },
 ];
 
 const basePlane = (id: string): PlaneState => ({
@@ -34,6 +34,7 @@ const basePackage = (id: string): PackageState => ({
   reward: 1234.5,
   cargoType: 'mail',
   cargoCount: 2,
+  weightKg: 600,
   deadlineSimTime: 0,
   status: 'open',
   planeId: null,