UTXO database model + startup performance optimizations

Replace per-transaction CTxIndex spent tracking with per-output UTXO
database (CUtxoEntry). ConnectBlock writes/erases UTXOs as blocks are
processed. FetchInputs reads directly from UTXO DB instead of
deserializing full transactions from disk.

Persist nChainTrust in block index (dbformat v3) to skip expensive
recalculation on every startup. Only populate setStakeSeen for last
500 blocks instead of all 2M+.

Lazy fallback to old CTxIndex path for databases upgrading from
pre-UTXO format - no big-bang migration required.

Fixes pre-existing bugs in introdialog.cpp (extra brace) and
net_bootstrap.cpp (namespace extern).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/qt/introdialog.cpp

@@ -307,7 +307,6 @@ bool IntroDialog::pickDataDirectory()
                 progress.setValue(100);
             }
         }
-    }
 
     return true;
 }