feat(pay): Phase D 归集 —— 气隙签名 sweep(#34/34A)

动钱代码,按"联网建/广播 + 离线签"气隙流程,离线签名独立验签防篡改:
- wallet: DecodeTronAddress/TronAddressBodyHex(base58check 解码校验)、AddressFromMnemonic(离线校验用)。
- tron/sign: TxID(sha256 raw_data)、SignRawData(secp256k1 → R||S||recid 65B)、RecoverAddressBody。
- tron/abi: ABIEncodeTransferParams(transfer(address,uint256) 参数)、keccak 地址体。
- tron/tx(联网): BuildTransfer(triggersmartcontract 建未签名)、TRC20Balance、Broadcast。
- cmd/sweep: plan/build/sign/broadcast 四段;sign 对每笔独立验:①重算 txid 防篡改 ②收款人+额 ABI
  内嵌 ③USDT 合约内嵌 ④派生地址==owner,任一不符拒签;助记词只经 PAY_SWEEP_MNEMONIC(离线机,不入 arg)。
- 测试:签名可恢复到正确地址、ABI 编码、地址解码 roundtrip 全绿。
- 联网上链部分标注需 Phase E 真链验证;README 补 sweep 用法 + 验签说明 + gas 提醒。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pay/internal/wallet/seed.go

@@ -84,3 +84,26 @@ func PrivKeyHexFromMnemonic(mnemonic, passphrase string, account, change, index
 	}
 	return fmt.Sprintf("%x", priv.Serialize()), nil
 }
+
+// AddressFromMnemonic derives the TRON address at m/44'/195'/<account>'/<change>/<index>
+// straight from the mnemonic. OFFLINE ONLY — used by the sweep signer to verify
+// that a derived key matches the address it is about to sign for.
+func AddressFromMnemonic(mnemonic, passphrase string, account, change, index uint32) (string, error) {
+	acct, err := AccountKeyFromMnemonic(mnemonic, passphrase, account)
+	if err != nil {
+		return "", err
+	}
+	chainKey, err := acct.Derive(change)
+	if err != nil {
+		return "", fmt.Errorf("wallet: derive change: %w", err)
+	}
+	addrKey, err := chainKey.Derive(index)
+	if err != nil {
+		return "", fmt.Errorf("wallet: derive index: %w", err)
+	}
+	pub, err := addrKey.ECPubKey()
+	if err != nil {
+		return "", fmt.Errorf("wallet: ec pubkey: %w", err)
+	}
+	return PubKeyToTronAddress(pub.SerializeUncompressed()), nil
+}