Add BIP39 seed phrases (SilentDragonXLite-compatible) and HD transparent keys

Derive transparent (t-addr) keys from the HD seed and add BIP39 mnemonic seed
phrases that are byte-for-byte compatible with SilentDragonXLite, so the same
24 words recover the same shielded and transparent addresses in either wallet.

HD transparent keys:
- Derive t-keys from the seed at m/44'/coin'/0'/0/i (were random CKeys).
- CHDChain gains a version-gated transparent counter; existing wallets load
  unchanged. GenerateNewKey routes through DeriveNewChildKey when enabled
  (-hdtransparent, default on).
- Restore from a seed hex via -hdseed with gap-limit pre-derivation; birthday
  pinned to genesis so the rescan is not clipped.

BIP39 seed phrases:
- Wire the vendored trezor BIP39 lib (src/crypto/bip39) into the build, fix its
  BIP39_WORDS guard, and disable the insecure mnemonic cache.
- Match SDXLite exactly: English wordlist, empty passphrase, PBKDF2 64-byte
  seed, coin type 141, ZIP-32 m/32'/141'/i' and BIP44 m/44'/141'/0'/0/i. Store
  the 32-byte entropy and expand to the 64-byte seed on demand.
- Restore via -mnemonic, create via -usemnemonic, reveal via z_exportmnemonic.

Verified by gtests including a known-answer BIP39 seed vector and z/t address
derivation checks (src/gtest/test_hdtransparent.cpp, test_mnemonic_compat.cpp).
Docs in doc/hd-transparent-keys.md and doc/seed-phrase.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/wallet/rpcdump.cpp

@@ -742,7 +742,9 @@ UniValue dumpwallet_impl(const UniValue& params, bool fHelp, bool fDumpZKeys)
     file << strprintf("#   mined on %s\n", EncodeDumpTime(chainActive.Tip()->GetBlockTime()));
     {
         HDSeed hdSeed;
-        pwalletMain->GetHDSeed(hdSeed);
+        // Dump the 64-byte derivation seed (for mnemonic wallets this is the
+        // expanded BIP39 seed), so re-importing the hex reproduces the same keys.
+        pwalletMain->GetHDSeedForDerivation(hdSeed);
         auto rawSeed = hdSeed.RawSeed();
         file << strprintf("# HDSeed=%s fingerprint=%s", HexStr(rawSeed.begin(), rawSeed.end()), hdSeed.Fingerprint().GetHex());
         file << "\n";
@@ -1026,6 +1028,50 @@ UniValue z_exportkey(const UniValue& params, bool fHelp, const CPubKey& mypk)
     return EncodeSpendingKey(sk.get());
 }
 
+UniValue z_exportmnemonic(const UniValue& params, bool fHelp, const CPubKey& mypk)
+{
+    if (!EnsureWalletIsAvailable(fHelp))
+        return NullUniValue;
+
+    if (fHelp || params.size() != 0)
+        throw runtime_error(
+            "z_exportmnemonic\n"
+            "\nReveal the wallet's BIP39 seed phrase (24 words).\n"
+            "The phrase is byte-compatible with SilentDragonXLite: the same words\n"
+            "restore the same transparent and shielded addresses in either wallet.\n"
+            "Only works for wallets created or restored from a mnemonic (see the\n"
+            "-mnemonic and -usemnemonic options). Requires the wallet be unlocked.\n"
+            "\nResult:\n"
+            "{\n"
+            "  \"mnemonic\" : \"word1 ... word24\",   (string) the BIP39 seed phrase\n"
+            "  \"seedfp\" : \"hex\"                    (string) the HD seed fingerprint\n"
+            "}\n"
+            "\nExamples:\n"
+            + HelpExampleCli("z_exportmnemonic", "")
+            + HelpExampleRpc("z_exportmnemonic", "")
+        );
+
+    LOCK2(cs_main, pwalletMain->cs_wallet);
+
+    EnsureWalletIsUnlocked();
+
+    if (!pwalletMain->IsMnemonicSeed()) {
+        throw JSONRPCError(RPC_WALLET_ERROR,
+            "This wallet's seed was not derived from a mnemonic, so no seed phrase is available. "
+            "Use z_exportwallet to back up the raw HD seed instead.");
+    }
+
+    std::string phrase;
+    if (!pwalletMain->GetMnemonicPhrase(phrase)) {
+        throw JSONRPCError(RPC_WALLET_ERROR, "Could not retrieve the seed phrase (is the wallet unlocked?)");
+    }
+
+    UniValue ret(UniValue::VOBJ);
+    ret.push_back(Pair("mnemonic", phrase));
+    ret.push_back(Pair("seedfp", pwalletMain->GetHDChain().seedFp.GetHex()));
+    return ret;
+}
+
 UniValue z_exportviewingkey(const UniValue& params, bool fHelp, const CPubKey& mypk)
 {
     if (!EnsureWalletIsAvailable(fHelp))