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>

Taskmaster/Tasks/task-4-8.md

@@ -5,7 +5,7 @@ priority: "medium"
 depends_on: ["2", "3", "4", "5", "6", "7"]
 parent_id: "4"
 source: "taskmaster"
-generated_at: "2026-05-24T07:26:29.052Z"
+generated_at: "2026-05-28T11:49:27.076Z"
 ---
 
 # 4.8 - Make backend-core stack decision
@@ -28,8 +28,6 @@ Choose whether the security-critical backend core remains TypeScript or moves to
 
 ## Details
 
-Completed. Produced `09 - Audits/Backend Core Stack Decision Record - 2026-05-24.md`.
-
 Evaluate team capability, two-year maintainability, operational footprint, rewrite cost, dual-stack complexity, auditability, supply-chain exposure, and which modules belong in a payment/auth/escrow core versus the existing marketplace/chat API.
 
 ## Verification