feat(pay): pangolin-pay 钱包派生模块(#34/34A · 加密货币交易引擎 Phase B.2)

新建独立 Go module pay/(pangolin-pay:独立 VPS 跑,与控制面分开、隔离 crypto 依赖)。
wallet 包:
- AddressFromAccountXpub(xpub, change, index):watch-only 从账户 xpub 派生 TRON 地址
  (Keccak-256 legacy → 后20字节 → 0x41 → base58check),watcher 用,不碰私钥。
- seed.go(离线专用):助记词→account xpub / 地址私钥(hex),给 Phase A 导 xpub、Phase D 归集签名。
- 测试:①派生一致性——xpub 路径与私钥路径逐个相等(证明每个收款地址对得上签名私钥);
  ②金标准向量锁定实现防回归。⚠️ 金标准需按 A.4 用 Ian Coleman 交叉核对一次。

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

pay/internal/wallet/seed.go

@@ -0,0 +1,86 @@
+package wallet
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btcd/btcutil/hdkeychain"
+	"github.com/btcsuite/btcd/chaincfg"
+	bip39 "github.com/tyler-smith/go-bip39"
+)
+
+// ⚠️ OFFLINE ONLY. Everything in this file touches the BIP39 seed / private keys.
+// It exists for (a) generating the account xpub to hand to the watcher, and
+// (b) deriving per-address private keys for offline sweep signing (Phase D).
+// It must NEVER be linked into or run on the internet-facing pangolin-pay
+// watcher — the hot service only ever handles the account xpub (see derive.go).
+
+const (
+	purposeBIP44 = 44
+	coinTypeTRON = 195
+	// hardenedOffset marks a derivation index as hardened (requires the private
+	// key). BIP44's first three levels (purpose'/coin'/account') are hardened.
+	hardenedOffset = hdkeychain.HardenedKeyStart // 0x80000000
+)
+
+// AccountKeyFromMnemonic derives the account-level extended *private* key at
+// m/44'/195'/<account>' from a BIP39 mnemonic (+ optional passphrase).
+// OFFLINE ONLY.
+func AccountKeyFromMnemonic(mnemonic, passphrase string, account uint32) (*hdkeychain.ExtendedKey, error) {
+	if !bip39.IsMnemonicValid(mnemonic) {
+		return nil, fmt.Errorf("wallet: invalid BIP39 mnemonic (checksum/wordlist)")
+	}
+	seed := bip39.NewSeed(mnemonic, passphrase)
+	master, err := hdkeychain.NewMaster(seed, &chaincfg.MainNetParams)
+	if err != nil {
+		return nil, fmt.Errorf("wallet: master key: %w", err)
+	}
+	for _, step := range []uint32{
+		hardenedOffset + purposeBIP44,
+		hardenedOffset + coinTypeTRON,
+		hardenedOffset + account,
+	} {
+		master, err = master.Derive(step)
+		if err != nil {
+			return nil, fmt.Errorf("wallet: derive account path: %w", err)
+		}
+	}
+	return master, nil
+}
+
+// AccountXpubFromMnemonic returns the account-level xpub string to hand to the
+// watcher. OFFLINE ONLY — run this once on the air-gapped machine, copy only the
+// returned xpub to the hot service.
+func AccountXpubFromMnemonic(mnemonic, passphrase string, account uint32) (string, error) {
+	k, err := AccountKeyFromMnemonic(mnemonic, passphrase, account)
+	if err != nil {
+		return "", err
+	}
+	pub, err := k.Neuter() // strip the private key -> xpub
+	if err != nil {
+		return "", fmt.Errorf("wallet: neuter: %w", err)
+	}
+	return pub.String(), nil
+}
+
+// PrivKeyHexFromMnemonic derives the raw secp256k1 private key (hex) for the
+// address at m/44'/195'/<account>'/<change>/<index>, for offline sweep signing.
+// OFFLINE ONLY.
+func PrivKeyHexFromMnemonic(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)
+	}
+	priv, err := addrKey.ECPrivKey()
+	if err != nil {
+		return "", fmt.Errorf("wallet: ec privkey: %w", err)
+	}
+	return fmt.Sprintf("%x", priv.Serialize()), nil
+}