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/DatServiceImpl.cs

@@ -55,22 +55,25 @@ namespace Decal.DecalDat
 
         private void LoadFilters()
         {
-            try
-            {
-                var config = _decalCore.Configuration;
-                if (config == null) return;
-
-                // Look up "FileFilters" collection in Decal configuration
-                // The C++ code iterates an IDecalEnum to read filter definitions
-                // Each filter has: Prefix (string), Cache (bool), ComClass (CLSID)
-                //
-                // For now, register the known built-in filters that ship with Decal.
-                // When Decal.Core is implemented, this will read from live config.
-            }
-            catch
-            {
-                // Config may not be available yet
-            }
+            // The C++ original calls:
+            //   m_pDecal->get_Configuration(L"FileFilters", GUID_NULL, &pEnum)
+            // then iterates the IDecalEnum reading:
+            //   pEnum->get_ComClass(&clsid)
+            //   pEnum->get_Property(L"Prefix", &vPrefix)   // protocol name, e.g. "portal"
+            //   pEnum->get_Property(L"Cache", &vCache)      // 0 or 1
+            //
+            // However, IDecalCore.Configuration and IDecalEnum.Property are
+            // parameterized COM properties that the decompiler flattened to
+            // simple getters. They need to be called via IDispatch to pass
+            // the category name / property name arguments.
+            //
+            // Filters are registered externally (by plugins/installer in the
+            // registry under HKLM\SOFTWARE\Decal\FileFilters\{CLSID}) and
+            // will be loaded once Decal.Core exposes the Configuration enum.
+            //
+            // DecalDat still works without filters — callers get raw IDatStream
+            // objects. Filters are an optional layer that parses the stream into
+            // typed objects (e.g., spell tables, character data).
         }
 
         public void BeforePlugins()