Add linescan plugin for line scan camera simulation

- New plugin extracts single row/column from frames and builds line scan image
- Supports horizontal mode (extract row, stack vertically)
- Supports vertical mode (extract column, stack horizontally)
- Configurable line index and output size
- Proper caps negotiation with fixate_caps implementation
- Updated build system to include linescan plugin
- Added comprehensive README with usage examples

gst/linescan/README.md

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+# GStreamer Linescan Plugin
+
+## Overview
+
+The `linescan` plugin simulates a line scan camera by extracting a single row or column of pixels from each input frame and stacking them to create a line scan image over time. This is particularly useful for analyzing fast-moving objects captured with high frame rate cameras (typically 200-750 fps).
+
+## Features
+
+- **Horizontal mode**: Extracts a single row from each frame and stacks them vertically
+- **Vertical mode**: Extracts a single column from each frame and stacks them horizontally
+- **Configurable line selection**: Choose which row/column to extract, or use the middle automatically
+- **Configurable output size**: Set the number of lines to accumulate before wrapping around
+- **Supports multiple pixel formats**: GRAY8, GRAY16_LE, GRAY16_BE, RGB, BGR, BGRA, RGBx
+
+## Properties
+
+| Property | Type | Default | Description |
+|----------|------|---------|-------------|
+| `direction` | enum | horizontal | Direction to extract line: `horizontal` (row) or `vertical` (column) |
+| `line-index` | int | -1 | Index of row/column to extract (-1 for middle of image) |
+| `output-size` | int | 800 | Number of lines to accumulate (width for horizontal mode, height for vertical mode) |
+
+## Usage Examples
+
+### Basic Horizontal Line Scan (Extract Row 100)
+```bash
+gst-launch-1.0 videotestsrc ! \
+    linescan direction=horizontal line-index=100 output-size=800 ! \
+    videoconvert ! autovideosink
+```
+
+### Vertical Line Scan (Extract Middle Column)
+```bash
+gst-launch-1.0 videotestsrc ! \
+    linescan direction=vertical line-index=-1 output-size=600 ! \
+    videoconvert ! autovideosink
+```
+
+### High-Speed Camera Line Scan
+```bash
+gst-launch-1.0 idsueyesrc framerate=500 ! \
+    videocrop bottom=3 ! \
+    linescan direction=horizontal line-index=50 output-size=1000 ! \
+    videoconvert ! autovideosink
+```
+
+### Save Line Scan to File
+```bash
+gst-launch-1.0 idsueyesrc config-file=config.ini framerate=200 ! \
+    linescan direction=horizontal output-size=2000 ! \
+    videoconvert ! \
+    pngenc ! filesink location=linescan.png
+```
+
+## How It Works
+
+### Horizontal Mode (default)
+1. Extracts one horizontal row from each input frame
+2. The row is determined by `line-index` (or middle if -1)
+3. Each extracted row is stacked vertically to build the output image
+4. Output dimensions: `output-size` (width) × `input_height` (height)
+5. When the buffer fills up (after `input_height` frames), it wraps around and starts overwriting from the top
+
+### Vertical Mode
+1. Extracts one vertical column from each input frame
+2. The column is determined by `line-index` (or middle if -1)
+3. Each extracted column is stacked horizontally to build the output image
+4. Output dimensions: `input_width` (width) × `output-size` (height)
+5. When the buffer fills up (after `input_width` frames), it wraps around and starts overwriting from the left
+
+## Typical Use Cases
+
+1. **Conveyor Belt Inspection**: Capture a continuous image of objects moving on a conveyor belt
+2. **High-Speed Object Analysis**: Analyze fast-moving objects frame-by-frame at high framerates
+3. **Barcode/Text Reading**: Extract a horizontal line across moving barcodes or text
+4. **Web Inspection**: Continuous inspection of paper, textile, or other web materials
+5. **Sports Analysis**: Track trajectory or movement patterns of fast-moving objects
+
+## Pipeline Tips
+
+### For Best Results
+- Use a high framerate camera (200-750 fps recommended)
+- Ensure consistent object motion
+- Adjust `line-index` to capture the region of interest
+- Set `output-size` based on expected duration or object size
+- Use `videocrop` before linescan if needed to reduce processing
+
+### Common Pipeline Patterns
+```bash
+# Pattern 1: Crop + Linescan + Display
+camera ! videocrop ! linescan ! videoconvert ! autovideosink
+
+# Pattern 2: Linescan + Encode + Save
+camera ! linescan ! videoconvert ! x264enc ! mp4mux ! filesink
+
+# Pattern 3: Linescan + Network Stream
+camera ! linescan ! videoconvert ! jpegenc ! multipartmux ! tcpserversink
+```
+
+## Performance Considerations
+
+- The plugin maintains an internal buffer equal to the full output image size
+- Memory usage = `output_width × output_height × bytes_per_pixel`
+- Processing overhead is minimal (single memcpy per frame)
+- No frame buffering - processes each frame immediately
+
+## Troubleshooting
+
+### "Could not link" errors
+- Ensure videocrop or other upstream elements provide fixed caps
+- Try adding `videoconvert` before linescan if needed
+
+### Line index out of range
+- Check that `line-index` is less than input height (horizontal) or width (vertical)
+- Use `-1` to automatically select the middle
+
+### Wrapping/Rolling effect
+- This is normal when the buffer fills up
+- Adjust `output-size` to match your capture duration needs
+
+## See Also
+
+- [intervalometer](../intervalometer/README.md) - Auto-exposure control
+- [rollingsum](../rollingsum/README.md) - Moving window sum detection
+- [GStreamer Documentation](https://gstreamer.freedesktop.org/documentation/)