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_source.h

@@ -0,0 +1,13 @@
+#pragma once
+#include "core.h"
+
+namespace rcx {
+
+// Import C/C++ struct definitions from source code into a NodeTree.
+// Supports two modes (auto-detected):
+//   1. With comment offsets (// 0xNN) - trusts the offset values
+//   2. Without comment offsets - computes offsets from type sizes
+// Returns an empty NodeTree on failure; populates errorMsg if non-null.
+NodeTree importFromSource(const QString& sourceCode, QString* errorMsg = nullptr);
+
+} // namespace rcx