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

03 - API Reference/Payment API.md

@@ -464,6 +464,42 @@ Same result shape as above, but for a single destination.
 
 Escrow state (`escrowState`): `unfunded` → `funded` → `released` | `refunded`.
 
+## Request Network multichain registry (admin)
+
+### `GET /api/admin/rn/networks`
+
+**Auth:** Admin only  
+**Response 200:**
+```json
+{
+  "success": true,
+  "data": [
+    {
+      "chainId": 56,
+      "name": "BNB Smart Chain",
+      "shortName": "BSC",
+      "rpcUrl": "https://bsc-dataseed.binance.org/",
+      "publicRpcUrl": "https://bsc-rpc.publicnode.com",
+      "blockExplorer": "https://bscscan.com",
+      "proxyAddress": "0x0DfbEe143b42B41eFC5A6F87bFD1fFC78c2f0aC9",
+      "nativeCurrency": { "name": "BNB", "symbol": "BNB", "decimals": 18 },
+      "confirmationThreshold": 12,
+      "tokens": [
+        { "address": "0x8ac76a51cc950d9822d68b83fe1ad97b32cd580d", "symbol": "USDC", "decimals": 18, "name": "Binance-Peg USD Coin" },
+        { "address": "0x55d398326f99059ff775485246999027b3197955", "symbol": "USDT", "decimals": 18, "name": "Binance-Peg BSC-USD" }
+      ]
+    }
+  ],
+  "meta": { "chainCount": 5, "tokenCount": 10 }
+}
+```
+
+### `POST /api/admin/rn/networks/reload`
+
+**Auth:** Admin only  
+**Description:** Reload the in-memory chain + token registry from JSON files on disk. Call after editing `supportedChains.json` or `tokens.json` in the container.  
+**Response 200:** `{ "success": true, "message": "Registry reloaded from disk" }`
+
 ## Related
 
 - [[Payment Flow]]