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/

third_party/raw_pdb/raw_pdb.natvis

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="PDB::ArrayView&lt;*&gt;">
+		<DisplayString>{{ size={m_length} }}</DisplayString>
+		<Expand>
+			<ArrayItems>
+				<Size>m_length</Size>
+				<ValuePointer>m_data</ValuePointer>
+			</ArrayItems>
+		</Expand>
+	</Type>
+</AutoVisualizer>