wallet: run the node on a SQLite backend (opt-in)

Adds -walletbackend=sqlite: an opt-in flag that runs the node on
<datadir>/wallet.sqlite instead of wallet.dat. It requires an export to
exist first (no silent migration) and prints a step-by-step guide when
one does not; wallet.dat is never opened or touched, so a plain restart
goes back to Berkeley DB.

- LoadWalletFromSQLiteRuntime opens the export read-write, keeps the
  handle open as the active backend, loads records into memory through
  the same visitor/reconcile/finish path as the read-only staging load,
  and never fabricates a default key.
- Shutdown checkpoints and closes the SQLite backend; a kill before that
  loses nothing because WAL + synchronous=FULL already made every
  committed write durable.
- Self-test proves runtime writes persist across a close and reopen.

BDB stays the default: nothing activates the SQLite runtime unless the
flag is given. One-shot audit/recovery commands still use Berkeley DB.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/db.h b/src/db.h
index 8fa4ea8..fee5a82 100644
--- a/src/db.h
+++ b/src/db.h
@@ -624,6 +624,7 @@ bool ScanWalletRecords(CWalletRecordVisitor& visitor, string& strErrorRet);
 
 bool LoadWallet();
 bool LoadWalletFromSQLite(const string& strPath);
+bool LoadWalletFromSQLiteRuntime(const string& strPath);
 
 // Writes a wallet.dat that opens on its own, anywhere -- see the definition in
 // db.cpp for why a plain file copy does not. Still a point-in-time snapshot: