release(csharp): minor bump to 0.12.0 (FR-024 feature + B4b breaking)

Minor (not patch) to signal the FR-024 feature surface + the B4b breaking change,
and to bring the C# line up onto 0.12.x alongside Python/npm.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LuZWKnWzYGVnESijL7uuky

Author: dmealing

server/csharp/Directory.Build.props

@@ -8,7 +8,7 @@
   -->
 
   <PropertyGroup>
-    <Version>0.11.2</Version>
+    <Version>0.12.0</Version>
     <Authors>Doug Mealing</Authors>
     <Company>Doug Mealing LLC</Company>
     <Product>MetaObjects</Product>