Fix MSBuild targets to preserve unprocessed source files

The previous implementation removed ALL source files from compilation
and only included rewritten files. This broke namespace resolution for
types in files without Assert() calls (like PipeMock).

This fix:
- Only removes files that were actually rewritten
- Includes both rewritten files AND unprocessed source files
- Preserves the complete compilation context

Fixes namespace resolution errors like:
CS0103: The name 'PipeMock' does not exist in the current context

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: yevhen

src/SharpAssert.IntegrationTests/ReferencedFilesFixture.cs

@@ -0,0 +1,16 @@
+using static SharpAssert.Sharp;
+
+namespace SharpAssert.IntegrationTests;
+
+[TestFixture]
+public class ReferencedFilesFixture
+{
+    [Test]
+    public void Test_can_use_code_in_non_rewritten_files()
+    {
+        var helper = new TestHelper();
+        helper.Increment();
+
+        Assert(helper.Count > 0);
+    }
+}

src/SharpAssert.IntegrationTests/TestHelper.cs

@@ -0,0 +1,7 @@
+namespace SharpAssert.IntegrationTests;
+
+public class TestHelper
+{
+    public int Count = 42;
+    public void Increment() => Count++;
+}

src/SharpAssert/build/SharpAssert.targets

@@ -41,12 +41,23 @@
     <ItemGroup>
       <!-- Track rewritten files for potential cleanup -->
       <_SharpRewrittenFiles Include="$(IntermediateOutputPath)SharpRewritten\**\*.sharp.g.cs" />
-      <!-- Only during actual build: swap original files with rewritten versions -->
-      <Compile Remove="@(_SharpSourceFiles)" />
+      
+      <!-- Identify which source files were actually rewritten -->
+      <_SharpProcessedSources Include="@(_SharpSourceFiles)" 
+        Condition="Exists('$(IntermediateOutputPath)SharpRewritten\%(RecursiveDir)%(Filename)%(Extension).sharp.g.cs')" />
+      
+      <!-- Find source files that weren't processed (no Assert calls) -->
+      <_SharpUnprocessedSources Include="@(_SharpSourceFiles)" 
+        Exclude="@(_SharpProcessedSources)" />
+      
+      <!-- Only remove files that were actually rewritten -->
+      <Compile Remove="@(_SharpProcessedSources)" />
+      
+      <!-- Include only rewritten files -->
       <Compile Include="@(_SharpRewrittenFiles)" />
     </ItemGroup>
 
-    <Message Text="SharpAssert: Replaced @(_SharpSourceFiles->Count()) source files with rewritten versions (actual build)" Importance="normal"
+    <Message Text="SharpAssert: Processed @(_SharpProcessedSources->Count()) of @(_SharpSourceFiles->Count()) source files, preserved @(_SharpUnprocessedSources->Count()) unmodified files" Importance="normal"
              Condition="'$(SharpAssertEmitRewriteInfo)'=='true'" />
 
   </Target>