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.

plugins/ProcessMemory/ProcessMemoryPlugin.h

@@ -28,6 +28,7 @@ public:
 
     bool isLive() const override { return true; }
     uint64_t base() const override { return m_base; }
+    QVector<rcx::MemoryRegion> enumerateRegions() const override;
     bool isReadable(uint64_t, int len) const override {
 #ifdef _WIN32
         return m_handle && len >= 0;