Add SVG combination tool and plate dimension options

Created combine_svg_vertical.py to vertically stack SVG files with centering
and configurable spacing. Enhanced svg_logo_to_stl.py with plate dimensions
and margin options for better nameplate creation.

combine_svg_vertical.py features:
- Vertically combines two SVG files (icon above text)
- Automatically centers smaller element
- Configurable spacing between elements (default: 20px)
- Configurable margin around composition (default: 10px)
- Preserves individual SVG structure in groups

svg_logo_to_stl.py enhancements:
- Added --plate-width and --plate-height options
- Logo scales to fit within plate dimensions (maintaining aspect ratio)
- Added --margin option for spacing around logo on plate
- Automatic centering of logo on plate
- Reports actual dimensions: plate, logo, margin, and offset

Example usage:
  # Combine icon and text
  python combine_svg_vertical.py um_grey.svg um_black.svg combined.svg --spacing 114

  # Center on 256x256mm plate with 10mm margin
  python svg_logo_to_stl.py combined.svg output \
    --plate-width 256 --plate-height 256 --margin 10 \
    --base-thickness 1.5 --feature-height 2.5

Files:
- combine_svg_vertical.py: Vertical SVG stacking tool
- README_combine_svg.md: Documentation for combine tool
- underground-magnetics-combined.svg: Example combined logo
- svg_logo_to_stl.py: Enhanced with plate dimensions
- command.txt: Updated with new parameters

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

README_combine_svg.md

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+# Combine SVG Vertical
+
+A script to combine two SVG files vertically with automatic centering and configurable spacing.
+
+## Purpose
+
+This tool is designed for preparing multi-part logos for 3D printing by:
+- Placing an icon/logo above text
+- Automatically centering the smaller element
+- Adding configurable spacing between elements
+- Maintaining proper dimensions for STL conversion
+
+## Usage
+
+```bash
+python combine_svg_vertical.py <top_svg> <bottom_svg> <output_svg> [options]
+```
+
+### Arguments
+
+- `top` - Top SVG file (e.g., icon or logo)
+- `bottom` - Bottom SVG file (e.g., text)
+- `output` - Output combined SVG file
+
+### Options
+
+- `--spacing, -s` - Vertical spacing between elements in pixels (default: 20)
+- `--margin, -m` - Margin around entire composition in pixels (default: 10)
+
+## Examples
+
+### Basic Usage (Underground Magnetics Logo)
+```bash
+python combine_svg_vertical.py um_grey.svg um_black.svg underground-magnetics-combined.svg
+```
+
+**Result:**
+- Grey "UM" icon (413×390) centered above black text (3070×233)
+- 20px spacing between icon and text
+- 10px margin around composition
+- Final canvas: 3090×663px
+
+### Custom Spacing
+```bash
+# Larger spacing between elements
+python combine_svg_vertical.py icon.svg text.svg combined.svg --spacing 50
+
+# Larger margin around composition
+python combine_svg_vertical.py icon.svg text.svg combined.svg --margin 30
+
+# Both custom
+python combine_svg_vertical.py icon.svg text.svg combined.svg -s 40 -m 20
+```
+
+## How It Works
+
+1. **Load SVGs** - Reads dimensions from viewBox or width/height attributes
+2. **Calculate Layout** - Determines positioning based on:
+   - Maximum width of both elements
+   - Combined height + spacing
+   - Centering offsets for smaller element
+3. **Create Groups** - Wraps each SVG in a `<g>` group with transform
+4. **Combine** - Creates new SVG canvas with both elements positioned
+
+## Output Structure
+
+```xml
+<svg width="..." height="..." viewBox="...">
+  <!-- Combined: top.svg (top) + bottom.svg (bottom) -->
+  <g id="top" transform="translate(x_offset, y_offset)">
+    <!-- All elements from top SVG -->
+  </g>
+  <g id="bottom" transform="translate(x_offset, y_offset)">
+    <!-- All elements from bottom SVG -->
+  </g>
+</svg>
+```
+
+## Integration with 3D Workflow
+
+After combining SVGs, you can convert to STL:
+
+```bash
+# Convert combined logo to 3D model
+python svg_logo_to_stl.py underground-magnetics-combined.svg um_logo \
+  --width-mm 100 \
+  --base-thickness 1.5 \
+  --feature-height 2.5
+```
+
+This will create a single 3D model with both the icon and text at the same height (not stacked vertically in 3D, just in the 2D layout).
+
+## Tips
+
+- **Spacing**: Use `--spacing` to adjust the gap between icon and text
+  - Small spacing (10-20): Compact look
+  - Medium spacing (20-40): Balanced (default)
+  - Large spacing (50+): Separated elements
+
+- **Margin**: Use `--margin` to add breathing room around the entire composition
+  - Useful when the combined SVG will be cropped or bounded
+
+- **Order Matters**: The first argument is always placed on top
+  - Icon above text: `combine_svg_vertical.py icon.svg text.svg ...`
+  - Text above icon: `combine_svg_vertical.py text.svg icon.svg ...`
+
+## File Size
+
+The combined SVG file size is approximately the sum of the input files:
+- um_grey.svg: 728B
+- um_black.svg: 6.1KB
+- **Combined**: 7.0KB