Treat warnings as errors repo-wide

A clean build currently has zero warnings; enforce that so new ones can't
accumulate unnoticed. Set TreatWarningsAsErrors in the root
Directory.Build.props rather than per project. ICSharpCode.ILSpyCmd keeps
its existing TreatWarningsAsErrors=false (project-level wins over the
imported default), and the three legacy net472 projects (the VS add-ins
and the installer builder) opt out explicitly since they only build on
Windows and aren't covered by the cross-platform CI that gates this.

Assisted-by: Claude:claude-opus-4-8:Claude Code

Directory.Build.props

@@ -3,6 +3,13 @@
     <NoWarn>$(NoWarn);NU1510</NoWarn>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <!-- Warnings are errors repo-wide so a clean build stays clean; individual projects can opt
+         back out (e.g. ICSharpCode.ILSpyCmd sets TreatWarningsAsErrors=false) since project-level
+         properties override this imported default. -->
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+
   <PropertyGroup>
     <EnableNUnitRunner>true</EnableNUnitRunner>
   </PropertyGroup>

ILSpy.AddIn.VS2022/ILSpy.AddIn.VS2022.csproj

@@ -11,6 +11,8 @@
     <Version>1.7.1.0</Version>
     <FileVersion>1.7.1.0</FileVersion>
     <LangVersion>9.0</LangVersion>
+    <!-- Legacy net472 add-in: opt out of the repo-wide warnings-as-errors (Directory.Build.props). -->
+    <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
 
     <EnableDefaultItems>False</EnableDefaultItems>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>

ILSpy.AddIn/ILSpy.AddIn.csproj

@@ -11,6 +11,8 @@
     <Version>1.7.1.0</Version>
     <FileVersion>1.7.1.0</FileVersion>
     <LangVersion>9.0</LangVersion>
+    <!-- Legacy net472 add-in: opt out of the repo-wide warnings-as-errors (Directory.Build.props). -->
+    <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
 
     <EnableDefaultItems>False</EnableDefaultItems>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>

ILSpy.Installer/ILSpy.Installer.csproj

@@ -4,6 +4,8 @@
     <OutputType>Exe</OutputType>
     <TargetFramework>net472</TargetFramework>
     <StartupObject>ILSpy.Installer.Builder</StartupObject>
+    <!-- Legacy net472 installer builder: opt out of the repo-wide warnings-as-errors (Directory.Build.props). -->
+    <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
   </PropertyGroup>
 
   <PropertyGroup>