v5.3.7: Fix sync stall, transaction display, balance updates, and bootstrap snapshots

Sync fixes:
- Extend stall detection beyond IBD to catch post-IBD sync gaps
- Walk-forward inv continuation to avoid CBlockLocator exponential gap loop
- Track walk-forward progress for stall recovery without restarting from scratch

GUI fixes:
- Load transactions synchronously in constructor (deferred QTimer never fired)
- Use beginResetModel/endResetModel instead of deprecated reset()
- Schedule full refresh on TRY_LOCK failure to avoid dropped CT_NEW notifications
- Only update cachedNumBlocks after successful balance check (prevents permanent loss)
- Add GetAllBalances() single-pass balance retrieval with TRY_LOCK

Bootstrap:
- Add trusted snapshot manifest verification for bootstrap archives
- Add IsKnownCheckpoint() to validate manifest against compiled-in checkpoints
- Skip txleveldb rebuild when verified manifest is present

Bump version to 5.3.7 across all packaging manifests.

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

src/checkpoints.cpp

@@ -60,6 +60,14 @@ namespace Checkpoints
         return hash == i->second;
     }
 
+    bool IsKnownCheckpoint(int nHeight, const uint256& hash)
+    {
+        MapCheckpoints& checkpoints = (fTestNet ? mapCheckpointsTestnet : mapCheckpoints);
+        MapCheckpoints::const_iterator i = checkpoints.find(nHeight);
+        if (i == checkpoints.end()) return false;
+        return hash == i->second;
+    }
+
     int GetTotalBlocksEstimate()
     {
         MapCheckpoints& checkpoints = (fTestNet ? mapCheckpointsTestnet : mapCheckpoints);