feat: PDB import via RawPDB, no msdia140.dll dependency

Replace DIA SDK COM-based PDB importer with RawPDB (MolecularMatters)
which reads PDB files directly via memory-mapped I/O. Adds File menu
"Import PDB..." dialog with type filtering, selection, and progress.

- Vendor raw_pdb into third_party/
- Two-phase API: enumeratePdbTypes() + importPdbSelected()
- Full recursive import of structs/unions/arrays/pointers/bitfields
- PDB import dialog with name filter, select-all, type count
- Benchmark: 1654 types from ntkrnlmp.pdb in 16ms
- Reorganize import/export files into src/imports/

src/imports/import_pdb.h

@@ -0,0 +1,34 @@
+#pragma once
+#include "core.h"
+#include <QVector>
+#include <functional>
+
+namespace rcx {
+
+struct PdbTypeInfo {
+    uint32_t typeIndex;      // TPI type index
+    QString  name;           // struct/class/union name
+    uint64_t size;           // sizeof in bytes
+    int      childCount;     // direct member count
+    bool     isUnion;        // union vs struct/class
+};
+
+// Phase 1: Enumerate all UDT types in the PDB (fast scan, no recursive import).
+QVector<PdbTypeInfo> enumeratePdbTypes(const QString& pdbPath,
+                                       QString* errorMsg = nullptr);
+
+// Phase 2: Import selected types with full recursive child types.
+// progressCb is called with (current, total) for each top-level type;
+// return false from the callback to cancel the import.
+using ProgressCb = std::function<bool(int current, int total)>;
+NodeTree importPdbSelected(const QString& pdbPath,
+                           const QVector<uint32_t>& typeIndices,
+                           QString* errorMsg = nullptr,
+                           ProgressCb progressCb = {});
+
+// Legacy single-call API: import one struct by name (or all if filter empty).
+NodeTree importPdb(const QString& pdbPath,
+                   const QString& structFilter = {},
+                   QString* errorMsg = nullptr);
+
+} // namespace rcx