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

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

step-07-wallet-api/serializer.ethereum-chain138.ts

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+/**
+ * Step 7 — Wallet API: serialize/deserialize for JSON-RPC
+ * Target: wallet-api packages/core/src/families/ethereum/serializer.ts
+ *
+ * Ethereum family already has serialization; ensure chainId is preserved.
+ * When adding a new family, implement serializeTransaction/deserializeTransaction
+ * for the new type and add to the union in packages/core/src/families/serializer.ts.
+ */
+
+import type { RawEthereumTransaction } from "./types.ethereum-chain138";
+import type { EthereumTransaction } from "./types.ethereum-chain138";
+
+export function serializeEthereumTransaction(
+  tx: EthereumTransaction
+): RawEthereumTransaction {
+  return {
+    family: "ethereum",
+    amount: tx.amount,
+    recipient: tx.recipient,
+    gasPrice: tx.gasPrice,
+    maxFeePerGas: tx.maxFeePerGas,
+    maxPriorityFeePerGas: tx.maxPriorityFeePerGas,
+    gasLimit: tx.gasLimit,
+    data: tx.data,
+    nonce: tx.nonce,
+    chainId: tx.chainId,
+  };
+}
+
+export function deserializeEthereumTransaction(
+  raw: RawEthereumTransaction
+): EthereumTransaction {
+  return {
+    family: "ethereum",
+    amount: raw.amount,
+    recipient: raw.recipient,
+    gasPrice: raw.gasPrice,
+    maxFeePerGas: raw.maxFeePerGas,
+    maxPriorityFeePerGas: raw.maxPriorityFeePerGas,
+    gasLimit: raw.gasLimit,
+    data: raw.data,
+    nonce: raw.nonce,
+    chainId: raw.chainId,
+  };
+}