fix(snap): preserve payload layout

Snapcraft strips the sole usr directory from tar sources, so extract the artifact directly and retain its desktop-file path.\n\nAdd a snap-only manual target to avoid rebuilding other platforms while validating packaging fixes.

snap/snapcraft.yaml

@@ -70,8 +70,10 @@ apps:
 
 parts:
   open-cad-studio:
-    plugin: dump
-    source: linux-payload.tar.gz
+    plugin: nil
+    override-build: |
+      mkdir -p "${CRAFT_PART_INSTALL}"
+      tar -xzf "${CRAFT_PROJECT_DIR}/linux-payload.tar.gz" -C "${CRAFT_PART_INSTALL}"
     stage-packages:
       - libxcursor1
       - libxi6