docs: Task #8 probe results + handoff + PRD AC updates

- Add Handoff - RN Multichain Probe - 2026-05-28.md
- Update Handoff - Request Network In-House Checkout with Task #8 status
- Update Activity Log with backend@ae17b18, frontend@0ebb2f1
- Update PRD §2 acceptance criteria for Task #8
- Update Payment API.md with /api/admin/rn/networks endpoints

PRD - Wallet, Multichain, Confirmations, AML, Trezor.md

@@ -129,11 +129,11 @@ Plus an admin "supported networks" UI that:
 - Cross-chain bridging.
 
 ### Acceptance criteria
-1. Probe script run on dev confirms RN proxy address on at least BSC, Arbitrum, Polygon, Ethereum, Base. Differences are documented.
-2. Token registry has entries for USDC + USDT on those 5 chains, with correct decimals.
-3. In-house checkout supports USDT on BSC end-to-end (a paid probe).
-4. USDT-mainnet `approve(0)` reset is handled in the UI when needed.
-5. Admin networks page renders the registry with a per-row "probe again" button.
+1. ✅ Probe script confirms RN proxy address on BSC, Arbitrum, Polygon, Ethereum. Base canonical address has no code — documented in [[08 - Operations/Handoff - RN Multichain Probe - 2026-05-28]].
+2. ✅ Token registry has entries for USDC + USDT on 4 verified chains (BSC, Arbitrum, Ethereum, Polygon) with correct decimals. Base entries removed pending proxy address discovery.
+3. 🔄 In-house checkout supports USDT on BSC end-to-end — **pending paid probe** (needs real wallet + test BSC USDT on dev).
+4. 🟡 USDT-mainnet `approve(0)` reset is handled in the UI — **implemented but unverified** (see handoff doc for Tenderly fork vs wait-for-usage decision).
+5. ✅ Admin networks page renders the registry with a reload-from-disk button.
 
 ---