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/providers/provider.h

@@ -1,11 +1,21 @@
 #pragma once
 #include <QByteArray>
 #include <QString>
+#include <QVector>
 #include <cstdint>
 #include <cstring>
 
 namespace rcx {
 
+struct MemoryRegion {
+    uint64_t base       = 0;
+    uint64_t size       = 0;
+    bool     readable   = true;
+    bool     writable   = false;
+    bool     executable = false;
+    QString  moduleName;
+};
+
 class Provider {
 public:
     virtual ~Provider() = default;
@@ -54,6 +64,11 @@ public:
         return 0;
     }
 
+    // Enumerate committed/readable memory regions.
+    // Used by the scan engine to know what address ranges to scan.
+    // Default: returns empty (scan engine falls back to [0, size())).
+    virtual QVector<MemoryRegion> enumerateRegions() const { return {}; }
+
     // --- Derived convenience (non-virtual, never override) ---
 
     bool isValid() const { return size() > 0; }