Feature: Add fig.canvas.download() for programmatic downloads

- Add public download() method to Canvas class
- Allows triggering downloads from Python code without clicking button
- Respects all savefig rcParams like the toolbar button
- Includes comprehensive docstring with examples
- Add test file demonstrating programmatic usage

Use cases:
- Batch downloading multiple figures
- Automated workflows
- Custom save logic in notebooks

Example:
    fig, ax = plt.subplots()
    ax.plot([1, 2, 3])
    fig.canvas.download()  # Triggers browser download

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

Author: ianhi

test_programmatic_download.py

@@ -0,0 +1,60 @@
+"""
+Test programmatic download using fig.canvas.download()
+
+This demonstrates the new public API for triggering downloads from Python code
+without clicking the toolbar button.
+"""
+
+# This would be run in a Jupyter notebook with %matplotlib ipympl
+import matplotlib
+matplotlib.use('module://ipympl.backend_nbagg')
+
+import matplotlib.pyplot as plt
+import numpy as np
+
+# Example 1: Simple programmatic download
+print("Example 1: Simple download")
+fig, ax = plt.subplots()
+ax.plot([1, 2, 3], [1, 4, 2])
+ax.set_title('Programmatic Download Test')
+
+# Trigger download programmatically - no button click needed!
+fig.canvas.download()
+print("  -> Downloads as PNG (default format)")
+
+# Example 2: Download as PDF
+print("\nExample 2: Download as PDF")
+plt.rcParams['savefig.format'] = 'pdf'
+
+fig, ax = plt.subplots()
+ax.plot(np.linspace(0, 10, 100), np.sin(np.linspace(0, 10, 100)))
+ax.set_title('PDF Download')
+
+fig.canvas.download()
+print("  -> Downloads as PDF")
+
+# Example 3: Batch download multiple figures
+print("\nExample 3: Batch download 3 figures")
+plt.rcParams['savefig.format'] = 'png'
+
+for i in range(3):
+    fig, ax = plt.subplots()
+    ax.plot(np.random.randn(50))
+    ax.set_title(f'Figure {i+1}')
+    fig.canvas.download()
+    print(f"  -> Downloaded Figure {i+1}")
+
+# Example 4: Download with custom settings
+print("\nExample 4: Custom DPI and transparent background")
+plt.rcParams['savefig.dpi'] = 150
+plt.rcParams['savefig.transparent'] = True
+
+fig, ax = plt.subplots()
+ax.scatter(np.random.randn(100), np.random.randn(100))
+ax.set_title('High-res Transparent')
+
+fig.canvas.download()
+print("  -> Downloads as 150 DPI PNG with transparent background")
+
+print("\n✅ All programmatic downloads triggered!")
+print("Check your Downloads folder for the files.")