build: ratchet warnings to errors for every project via Directory.Build.props; refresh stale comment (lmxopcua#766)

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

Directory.Build.props

@@ -2,16 +2,18 @@
   <!--
     Defaults inherited by every csproj. Individual projects may override.
 
-    TreatWarningsAsErrors is NOT set globally because test projects carry analyzer warnings
-    (e.g. xUnit1051) that would fail the build. src/ projects opt in to
-    <TreatWarningsAsErrors>true</TreatWarningsAsErrors> in their own csproj — all 61 do as of #606.
-    It stays per-project rather than global only because the test projects are not yet clean.
+    TreatWarningsAsErrors is ON for every project, src/ and tests alike (#766 ratchet). The solution builds with
+    0 warnings since #725 (xUnit1051 etc. cleaned), so any new warning, including analyzer ones, now fails the
+    build instead of accumulating. The 61 src/ projects also still set it in their own csproj (#606); that is
+    redundant but harmless. A project that genuinely needs an exception should suppress the specific warning
+    via NoWarn, not turn this off.
   -->
   <PropertyGroup>
     <TargetFramework>net10.0</TargetFramework>
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <LangVersion>latest</LangVersion>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 
   <!--