fix: strip nested brands in PayloadOf

[jo16oh]

PayloadOf stopped at a nested Val, so a handler returning PayloadOf<Order> still sent Money's brand to the client:

type Order = Val<"Order", { id: string; total: Money }>;
const body: PayloadOf<Order> = order;
const total: Money = body.total; // compiled, although nothing sealed it on this side

• open every nested Val and every Rec to its payload in PayloadOf
• Val.unwrap returns PayloadOf<V>, so its nested Vals are plain data too, as they are at run time
• docs: say both strip the brand at every depth

Code that stops compiling

The old types claimed a brand the value never had, so these relied on the bug:

• Order(Val.unwrap(order)) and Val.of(Val.unwrap(order)), when the payload holds a nested Val. Seal each nested Val first: Order({ ...raw, total: Money(raw.total) })
• Patch<PayloadOf<V>> passed to patch. Use Patch<SeedOf<V>>