WIP: modernization applied to DNS2 tree

Brings in uncommitted work from SAMI-PC E:\repos\triangles
cpp20-modernization branch:
- RocksDB default chain DB + auto-migrate from txleveldb
- SQLite default wallet + non-destructive migration from Berkeley
- Boost.Asio removed from RPC (rpc_httpsocket.h)
- Boost removed from all daemon + GUI code
- New: walletdb-base.h, walletdb-batch.h, walletdb-sqlite.{h,cpp},
  walletdb-factory.{h,cpp}, walletmigrate.{h,cpp}
- New tests: chaindb_runtime_tests, chaindb_equivalence_tests,
  snapshotnet_tests
- Docs: BOOST-REMOVAL.md, ROCKSDB-DEFAULT-MIGRATION.md,
  WALLET-SQLITE-MIGRATION.md

Does not yet build — needs CWalletDB->CWalletBatchTyped rebase in
walletdb.cpp/wallet.cpp/db.cpp and merge with origin/master for
v6 source files (checkpointpublisher, tor/, snapshot/, utxosnapshot,
bootstrap.cpp).

Build flags: -DBUILD_QT=OFF -DUSE_I2P_EMBEDDED=OFF

src/walletdb-factory.cpp

@@ -0,0 +1,55 @@
+// Copyright (c) 2026 The Triangles developers.
+// Distributed under the MIT/X11 software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "walletdb-base.h"
+#include "walletdb-sqlite.h"
+#include "util.h"
+
+#include <cctype>
+#include <filesystem>
+#include <stdexcept>
+#include <string>
+
+namespace fs = std::filesystem;
+
+WalletDbKind ResolveWalletDbKind()
+{
+    // SQLite is the default wallet backend. Berkeley DB is retained for one
+    // release as a fallback (-walletdb=bdb) and as the migration source.
+    std::string s = GetArg("-walletdb", std::string("sqlite"));
+    for (auto& c : s) c = std::tolower(static_cast<unsigned char>(c));
+
+    if (s == "sqlite")
+        return WalletDbKind::SQLite;
+    if (s == "bdb" || s == "berkeley")
+        return WalletDbKind::Berkeley;
+
+    throw std::runtime_error(
+        "-walletdb=" + s + " is not a recognized wallet backend. "
+        "Valid values: sqlite, bdb.");
+}
+
+std::unique_ptr<WalletDatabase> MakeWalletDatabase(const std::string& strFilename,
+                                                   std::string& strError)
+{
+    const fs::path path = GetDataDir() / strFilename;
+
+    switch (ResolveWalletDbKind()) {
+    case WalletDbKind::SQLite: {
+        auto db = std::make_unique<SQLiteDatabase>(path);
+        if (!db->Open(strError))
+            return nullptr;
+        return db;
+    }
+    case WalletDbKind::Berkeley:
+        // The Berkeley backend is still served by the legacy CWalletDB/CDB code
+        // path. The thin BerkeleyDatabase adapter that plugs the existing
+        // CDBEnv/CDB into this seam is added during CWalletDB integration; see
+        // WALLET-SQLITE-MIGRATION.md. Until then, selecting -walletdb=bdb keeps
+        // the original code path rather than routing through MakeWalletDatabase.
+        strError = "Berkeley backend uses the legacy wallet path; not served by MakeWalletDatabase yet.";
+        return nullptr;
+    }
+    return nullptr;  // unreachable
+}