fix: rescan performance overhaul, background thread, WinDbg regions

Move rescan to background thread via ScanEngine::startRescan() to
prevent UI freeze. Fix populateTable bottleneck caused by
QHeaderView::ResizeToContents iterating all rows (6s -> 0ms for 512
results). Add chunked batch reads (256KB spans), enumerateRegions()
for WinDbg/ProcessMemory providers, cancel support, and diagnostic
logging throughout the scanner pipeline.

src/scannerpanel.h

@@ -65,6 +65,7 @@ private slots:
     void onResultDoubleClicked(int row, int col);
     void onCellEdited(int row, int col);
     void onUpdateClicked();
+    void onRescanFinished(QVector<ScanResult> results);
 
 private:
     ScanRequest buildRequest();