Fix DecalDat to work with real AC DAT files

The DAT file reader had several bugs inherited from the old C++ reference
code, which targeted an older format version. Verified and fixed against
real client_portal.dat and client_cell_1.dat files:

- Fix header offset: BTree root is at 0x160, not 0x148 (file size field)
- Fix BTree entry size: 24 bytes (flags+id+offset+size+timestamp), not 12
- Fix sector-chain node reading: BTree nodes span multiple sectors via
  linked-list headers; must assemble node data across sector boundaries
- Fix DatStreamImpl.Read() BSTR handling: use Buffer.BlockCopy to match
  C++ SysAllocStringByteLen instead of Marshal.PtrToStringAnsi
- Fix DatStreamImpl.ReadBinary() pointer lifetime: inline fixed block to
  keep destination buffer pinned during Marshal.Copy
- Document LoadFilters() dependency on parameterized COM properties in
  IDecalCore.Configuration that need IDispatch to call correctly

Add smoke test project (13/13 tests pass against real DAT files).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Managed/Decal.DecalDat.Tests/Decal.DecalDat.Tests.csproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <AssemblyName>Decal.DecalDat.Tests</AssemblyName>
+    <RootNamespace>Decal.DecalDat.Tests</RootNamespace>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\Decal.DecalDat\Decal.DecalDat.csproj" />
+    <ProjectReference Include="..\Decal.Interop.Dat\Decal.Interop.Dat.csproj" />
+  </ItemGroup>
+</Project>