chore: add unique assets from other branches into dan_branch

Bring hardware/spec content from drho1y-mvp_1 (3d_models, arduino_code, backend_control, kicad, specification) so dan_branch holds the shared union of branch files without rewriting other branch tips.

Co-authored-by: Cursor <cursoragent@cursor.com>

arduino_code/Test/.vscode/extensions.json

@@ -0,0 +1,10 @@
+{
+    // See http://go.microsoft.com/fwlink/?LinkId=827846
+    // for the documentation about the extensions.json format
+    "recommendations": [
+        "platformio.platformio-ide"
+    ],
+    "unwantedRecommendations": [
+        "ms-vscode.cpptools-extension-pack"
+    ]
+}