Add full Chain 138 integration: 8 steps, chain spec, app-ethereum config, docs

Co-authored-by: Cursor <cursoragent@cursor.com>

step-05-api/README.md

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+# Step 5 — API
+
+Provide **RPC and explorer** details to Ledger. Use **rpc-explorer-config.ts**:
+
+- **RPC_ENDPOINTS** — Public HTTPS and WSS URLs for Chain 138.
+- **EXPLORER** — Blockscout at https://explorer.d-bis.org (EIP3091); optional apiUrl for indexer.
+- **BLOCK_AVG_TIME_SEC** — ~2s for Chain 138 (Besu QBFT).
+
+Ledger may run their own indexer or proxy; they will specify. Have these values ready for their config or env (e.g. `API_DEFI_ORACLE_META_NODE`).

step-05-api/rpc-explorer-config.ts

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+/**
+ * Step 5 — API: RPC and Explorer config for Ledger Wallet / coin-module
+ * Target: ledger-live config or env; Ledger may use this for their indexer/RPC.
+ *
+ * Chain 138 public endpoints. Provide these to Ledger when they ask for API/RPC.
+ */
+
+export const CHAIN_ID = 138;
+
+export const RPC_ENDPOINTS = [
+  "https://rpc-http-pub.d-bis.org",
+  "wss://rpc-ws-pub.d-bis.org",
+  "https://rpc.d-bis.org",
+  "wss://rpc.d-bis.org",
+  "https://rpc2.d-bis.org",
+  "wss://ws.rpc.d-bis.org",
+  "wss://ws.rpc2.d-bis.org",
+  "https://rpc.public-0138.defi-oracle.io",
+  "wss://rpc.public-0138.defi-oracle.io",
+  "https://rpc.defi-oracle.io",
+  "wss://wss.defi-oracle.io",
+] as const;
+
+export const EXPLORER = {
+  name: "Blockscout",
+  url: "https://explorer.d-bis.org",
+  standard: "EIP3091",
+  apiUrl: "https://explorer.d-bis.org/api",
+} as const;
+
+export const BLOCK_AVG_TIME_SEC = 2;