Create Phase1Example sandbox with minimal test model

- New sandbox: Phase1Example.sandbox with proper module descriptor
- Test model: Phase1Test.mps with minimal Phase 1 concept usage
- Registry includes Module and Variable with proper child relationships
- Child relationships mapped: variables (2Ks9RG) and functions (2Ks9RH)
- Simple structure: Module containing one Variable

Avoids complex nested structures to focus on verifying basic concept resolution.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Phase1Example.sandbox/Phase1Example.sandbox.msd

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<solution name="Phase1Example.sandbox" uuid="a1b2c3d4-e5f6-47a8-9b0c-d1e2f3a4b5c6" moduleVersion="0">
+  <models>
+    <modelRoot contentPath="${module}" type="default">
+      <sourceRoot path="${module}/models" />
+    </modelRoot>
+  </models>
+  <facets>
+    <facet type="java" compile="off" classes="off" ext="no">
+      <classes generated="true" />
+    </facet>
+  </facets>
+  <languageVersions>
+    <language slang="l:a452e1cf-6f8c-4bca-8447-fd4d6e4d0003:SemAnno" version="0" />
+    <language slang="l:ceab5195-25ea-4f22-9b92-103b95ca8c0c:jetbrains.mps.lang.core" version="2" />
+  </languageVersions>
+  <dependencyVersions>
+    <module reference="a1b2c3d4-e5f6-47a8-9b0c-d1e2f3a4b5c6(Phase1Example.sandbox)" version="0" />
+  </dependencyVersions>
+</solution>