wallet: persist the mnemonic entropy in wallet.dat

Adds the record pair for the entropy, mirroring "hdseed"/"chdseed": a plaintext
form, an encrypted form that erases its plaintext counterpart the way
WriteCryptedKey does, and an erase.

Both new types are registered in IsKeyType so -salvagewallet preserves them.
Without that, salvage would silently drop the phrase while keeping the wallet
otherwise intact.

The records are defined but nothing writes them yet.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/wallet/walletdb.cpp

@@ -871,6 +871,10 @@ static bool IsKeyType(string strType)
 {
     return (strType == "key" || strType == "wkey" ||
             strType == "hdseed" || strType == "chdseed" ||
+            // The mnemonic entropy must survive a keys-only salvage: without it
+            // a recovered wallet keeps its seed (and stays fully spendable) but
+            // silently loses the ability to reprint its seed phrase.
+            strType == "mnementropy" || strType == "cmnementropy" ||
             strType == "zkey" || strType == "czkey" ||
             strType == "sapzkey" || strType == "csapzkey" ||
             strType == "vkey" ||
@@ -1345,6 +1349,24 @@ bool CWalletDB::EraseHDSeed(const uint256& seedFp)
     return Erase(std::make_pair(std::string("hdseed"), seedFp));
 }
 
+bool CWalletDB::WriteMnemonicEntropy(const uint256& entropyFp, const RawHDSeed& entropy)
+{
+    nWalletDBUpdated++;
+    return Write(std::make_pair(std::string("mnementropy"), entropyFp), entropy);
+}
+
+bool CWalletDB::WriteCryptedMnemonicEntropy(const uint256& entropyFp, const std::vector<unsigned char>& vchCryptedSecret)
+{
+    nWalletDBUpdated++;
+    return Write(std::make_pair(std::string("cmnementropy"), entropyFp), vchCryptedSecret);
+}
+
+bool CWalletDB::EraseMnemonicEntropy(const uint256& entropyFp)
+{
+    nWalletDBUpdated++;
+    return Erase(std::make_pair(std::string("mnementropy"), entropyFp));
+}
+
 bool CWalletDB::WriteHDChain(const CHDChain& chain)
 {
     nWalletDBUpdated++;