wallet: add an optional mnemonic-entropy secret to the key stores

Additive plumbing for storing a BIP39 entropy alongside the HD seed, mirroring
how the seed itself is handled through both key store layers: a plaintext member
on CBasicKeyStore, an encrypted pair on CCryptoKeyStore, encryption during the
unencrypted-to-encrypted conversion in EncryptKeys with the plaintext cleared,
and decryption on unlock.

RawHDSeed and CKeyingMaterial are the same secure_allocator vector type, so the
entropy passes through EncryptSecret/DecryptSecret with no adaptation.

Nothing calls this yet; there is no behaviour change.

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

src/keystore.cpp

@@ -68,6 +68,42 @@ bool CBasicKeyStore::GetHDSeed(HDSeed& seedOut) const
     }
 }
 
+bool CBasicKeyStore::SetMnemonicEntropy(const RawHDSeed& entropy)
+{
+    LOCK(cs_SpendingKeyStore);
+    if (entropy.empty()) {
+        // Never "install" nothing: HaveMnemonicEntropy() would stay false while
+        // the caller was told the call succeeded.
+        return false;
+    }
+    if (!mnemonicEntropy.empty()) {
+        // Same refuse-to-replace rule as SetHDSeed above, for a sharper reason:
+        // this is the printable form of the seed. If it could be swapped while
+        // hdSeed stayed put, the wallet would print a seed phrase that does not
+        // restore it -- strictly worse than printing none.
+        return false;
+    }
+    mnemonicEntropy = entropy;
+    return true;
+}
+
+bool CBasicKeyStore::HaveMnemonicEntropy() const
+{
+    LOCK(cs_SpendingKeyStore);
+    return !mnemonicEntropy.empty();
+}
+
+bool CBasicKeyStore::GetMnemonicEntropy(RawHDSeed& entropyOut) const
+{
+    LOCK(cs_SpendingKeyStore);
+    if (mnemonicEntropy.empty()) {
+        return false;
+    } else {
+        entropyOut = mnemonicEntropy;
+        return true;
+    }
+}
+
 bool CBasicKeyStore::AddKeyPubKey(const CKey& key, const CPubKey &pubkey)
 {
     LOCK(cs_KeyStore);