wallet: erase the plaintext HD seed record when the wallet is encrypted

CWalletDB::WriteCryptedHDSeed wrote the "chdseed" record and left "hdseed" in
place, unlike WriteCryptedKey which erases "key"/"wkey" after writing "ckey".
No erase of "hdseed" existed anywhere in src/wallet/.

CDB::Rewrite does not save us: EncryptWallet calls it with pszSkip defaulted, so
it copies every surviving record verbatim into the new file. The result is that a
wallet created unencrypted and later encrypted keeps its raw HD seed in cleartext
on disk permanently, and reloads it into memory on every start.

Add CWalletDB::EraseHDSeed and call it from CWallet::SetCryptedHDSeed after the
encrypted record is written, through the same CWalletDB so it shares
EncryptWallet's transaction. Erase returns true on DB_NOTFOUND, so a wallet that
was never written in plaintext is unaffected.

The erase is deliberately best-effort and only logs on failure. A hard failure
here propagates into CCryptoKeyStore::EncryptKeys, which EncryptWallet turns into
assert(false) with half the keys encrypted in memory; a warning is strictly
better than that.

Note this path is only reachable with -developerencryptwallet, which is
experimental and off by default on this chain, so this is a latent fix rather
than a live one.

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

src/wallet/walletdb.cpp

@@ -1335,6 +1335,16 @@ bool CWalletDB::WriteCryptedHDSeed(const uint256& seedFp, const std::vector<unsi
     return Write(std::make_pair(std::string("chdseed"), seedFp), vchCryptedSecret);
 }
 
+bool CWalletDB::EraseHDSeed(const uint256& seedFp)
+{
+    nWalletDBUpdated++;
+    // CDB::Erase honours activeTxn, so when this runs inside EncryptWallet's
+    // transaction the erase commits or aborts atomically with the chdseed write.
+    // It also returns true for DB_NOTFOUND, so erasing a record that was never
+    // written (e.g. a wallet encrypted at creation time) is not a failure.
+    return Erase(std::make_pair(std::string("hdseed"), seedFp));
+}
+
 bool CWalletDB::WriteHDChain(const CHDChain& chain)
 {
     nWalletDBUpdated++;