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.

app/api/packages/[id]/load/route.ts

@@ -1,10 +1,11 @@
 import { loadPackage, performAction } from '@/lib/game/actions';
 
 // POST /api/packages/:id/load (v0.12.0) — body { planeId: string }. Loads an OPEN
-// package onto a GROUNDED plane at the package's origin with a free slot. 400
-// missing/non-string planeId or unknown fields; 404 unknown package/plane id; 409
-// wrong package status, plane flying, plane not at the origin, no free slots. On
-// success returns the full state snapshot (package now "loaded" on that plane).
+// package onto a GROUNDED plane at the package's origin with enough payload room
+// for its weight (v0.27.0). 400 missing/non-string planeId or unknown fields; 404
+// unknown package/plane id; 409 wrong package status, plane flying, plane not at
+// the origin, no payload room. On success returns the full state snapshot
+// (package now "loaded" on that plane).
 
 export async function POST(req: Request, ctx: { params: Promise<{ id: string }> }) {
   const { id } = await ctx.params;