✨ Add structural organization headers to 04_layers module

- Added ## 🔧 DEVELOPMENT section before Step 1 where development begins
- Added ## 🤖 AUTO TESTING section before nbgrader block
- Updated to ## 🎯 MODULE SUMMARY: Neural Network Layers

Improves notebook organization without changing any code logic or content.

modules/source/04_layers/layers_dev.py

@@ -139,6 +139,11 @@ Each layer transforms the representation to be more useful for the final task.
 3. **Integration**: How layers work with activations and tensors
 """
 
+# %% [markdown]
+"""
+## 🔧 DEVELOPMENT
+"""
+
 # %% [markdown]
 """
 ## Step 1: Matrix Multiplication - The Engine of Neural Networks
@@ -680,6 +685,11 @@ Time to test your implementation! This section uses TinyTorch's standardized tes
 **This testing section is locked** - it provides consistent feedback across all modules and cannot be modified.
 """
 
+# %% [markdown]
+"""
+## 🤖 AUTO TESTING
+"""
+
 # %% nbgrader={"grade": false, "grade_id": "standardized-testing", "locked": true, "schema_version": 3, "solution": false, "task": false}
 # =============================================================================
 # STANDARDIZED MODULE TESTING - DO NOT MODIFY
@@ -694,7 +704,7 @@ if __name__ == "__main__":
 
 # %% [markdown]
 """
-## 🎯 Module Summary: Neural Network Layers Mastery!
+## 🎯 MODULE SUMMARY: Neural Network Layers
 
 Congratulations! You've successfully implemented the fundamental building blocks of neural networks: