release(csharp): MetaObjects 0.15.7 — FQN-exact nested @objectref (ADR-0041, #182)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013VXidfNN755CyTZTg1w8UV

Author: dmealing

server/csharp/Directory.Build.props

@@ -8,7 +8,7 @@
   -->
 
   <PropertyGroup>
-    <Version>0.15.6</Version>
+    <Version>0.15.7</Version>
     <Authors>Doug Mealing</Authors>
     <Company>Doug Mealing LLC</Company>
     <Product>MetaObjects</Product>