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/mainwindow.h

@@ -2,6 +2,7 @@
 #include "controller.h"
 #include "titlebar.h"
 #include "pluginmanager.h"
+#include "scannerpanel.h"
 #include <QMainWindow>
 #include <QMdiArea>
 #include <QMdiSubWindow>
@@ -152,6 +153,13 @@ private:
     void rebuildWorkspaceModel();
     void updateBorderColor(const QColor& color);
 
+    // Scanner dock
+    QDockWidget*          m_scannerDock      = nullptr;
+    ScannerPanel*         m_scannerPanel     = nullptr;
+    QLabel*               m_scanDockTitle    = nullptr;
+    QToolButton*          m_scanDockCloseBtn = nullptr;
+    void createScannerDock();
+
 protected:
     void changeEvent(QEvent* event) override;
     void resizeEvent(QResizeEvent* event) override;