docs: ship in-house RN checkout, scope 5 follow-up tasks (#7-11)

In-house Request Network checkout went fully end-to-end on dev today.
A real 0.01 USDC payment flowed through wallet connect -> approve ->
ERC20FeeProxy.transferFromWithReferenceAndFee -> RN webhook ->
TransactionSafetyProvider -> Payment.status=completed -> page success
state. Tx 0x494c77a29161b5100d8e0b1ac675f1822955d0bb3633ecdbfafb886f84f2f320.

Docs:
- New PRD: Wallet, Multichain, Confirmations, AML, Trezor
  (5 follow-ups, each sized for an independent contributor)
- Updated PRD: Request Network In-House Checkout (phases 0..3 done,
  phase 4 partial, phases 5-6 not started)
- Updated handoff: deployed versions, what is working end-to-end,
  follow-up tasks index

Taskmaster: 5 new top-level tasks (#7..#11) covering ephemeral
destination wallets, multichain proxy registry + USDC/USDT, runtime
confirmation thresholds, optional seller-paid AML screening, and
Trezor signing for admin actions. Tasks are scoped fine-grained so
each is independent enough for kimi to pick up.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -208,6 +208,7 @@ These are documented in their respective sections but worth highlighting:
 > - Opening a dispute does **not pause** the escrow until admin intervention. See [[Dispute Flow]] + [[Escrow Flow]].
 > - Several development env values committed as public — see [[Environment Variables]] for rotation list.
 > - Single-host deployment; horizontal scaling requires Redis adapter for Socket.IO — see [[Real-time Layer]] §8.
+> - Request Network webhooks currently land on the main app. Roadmap: Cloudflare Worker durable ingress + replay, with backend Transaction Safety Provider checks before escrow is credited. See [[Request Network Integration Constraints]].
 
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