Finish port of WindowsDirectory using P/Invoke (#1342)

Complete the previously-excluded WindowsDirectory port from Lucene 4.8.1,
replacing the original Java + C++/JNI native layer with Win32 P/Invoke
(CreateFileW with FILE_FLAG_RANDOM_ACCESS, ReadFile with positioned
NativeOverlapped reads, GetFileInformationByHandle, and SafeFileHandle for
the handle lifecycle). Modernized to current Lucene.NET conventions
(ReadInternal(Span<byte>), Position, DirectoryInfo/string ctors, Dispose
pattern) to match SimpleFSDirectory.

The read handle uses FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE.
Upstream 4.8.1 used only READ | WRITE; FILE_SHARE_DELETE is added to match the
SimpleFSDirectory read-path behavior from #1283 so an open read handle does
not block deletion of the underlying file on Windows.

The type is guarded to Windows via a runtime Constants.WINDOWS check
(throwing PlatformNotSupportedException) and [SupportedOSPlatform("windows")]
(under FEATURE_SUPPORTEDOSPLATFORMATTRIBUTE). Enabled AllowUnsafeBlocks in
Lucene.Net.Misc for the fixed read buffer, and narrowed the Store compile
exclusion to only the still-unfinished NativeUnixDirectory/NativePosixUtil.

Added test coverage (the original Lucene type had none): TestWindowsDirectory
runs the full BaseDirectoryTestCase suite (Windows-gated), plus bespoke tests
for random-access positioned reads, independent clone reads, read-past-EOF,
clone dispose not closing the shared handle, and concurrent multi-threaded
clone reads. A cross-platform fixture verifies the platform guard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: paulirwin

src/Lucene.Net.Misc/Lucene.Net.Misc.csproj

@@ -36,12 +36,16 @@
     <PackageTags>$(PackageTags);miscellaneous</PackageTags>
     <DocumentationFile>bin\$(Configuration)\$(TargetFramework)\$(AssemblyName).xml</DocumentationFile>
     <NoWarn>$(NoWarn);1591;1573</NoWarn>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
 
 
   <ItemGroup>
-    <Compile Remove="Store\*" />
+    <!-- LUCENENET TODO: The Unix/POSIX native directory port (NativeUnixDirectory) is not yet
+         complete and is excluded from compilation. See GH-1342. WindowsDirectory has been ported. -->
+    <Compile Remove="Store\NativeUnixDirectory.cs" />
+    <Compile Remove="Store\NativePosixUtil.cs" />
   </ItemGroup>
 
   <ItemGroup>