Add SKSignaturePadView with pressure-sensitive ink rendering

docs/docs/sksignaturepadview.md

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+# SKSignaturePadView
+
+The signature pad view provides fluid, pressure-sensitive ink rendering for capturing signatures or handwritten input. It supports stylus/pen pressure for variable stroke width (harder pressure = thicker lines) and uses quadratic Bezier curves to smooth jagged input from finger or pen movement.
+
+## Properties
+
+The main properties of a signature pad view are:
+
+| Property               | Type      | Description |
+| :--------------------- | :-------- | :---------- |
+| **StrokeColor**        | `Color`   | The color of the ink strokes. Default is `Black`. |
+| **MinStrokeWidth**     | `float`   | The minimum stroke width at zero pressure. Default is `1`. |
+| **MaxStrokeWidth**     | `float`   | The maximum stroke width at full pressure. Default is `8`. |
+| **PadBackgroundColor** | `Color`   | The background color of the signature pad. Default is `White`. |
+| **IsBlank**            | `bool`    | A read-only value indicating whether the pad has no strokes. |
+| **StrokeCount**        | `int`     | The number of strokes on the signature pad. |
+
+## Methods
+
+The signature pad provides several methods for manipulating and exporting signatures:
+
+| Method       | Return Type | Description |
+| :----------- | :---------- | :---------- |
+| **Clear()**  | `void`      | Clears all strokes from the signature pad. |
+| **Undo()**   | `bool`      | Removes the last stroke. Returns `true` if a stroke was removed. |
+| **ToPath()** | `SKPath?`   | Gets a combined path of all strokes. Returns `null` if blank. |
+| **ToImage(int width, int height, SKColor? backgroundColor)** | `SKImage?` | Renders the signature to an image with the specified dimensions. |
+| **GetStrokeBounds()** | `SKRect` | Gets the bounding rectangle of all strokes. |
+
+## Events
+
+| Event               | Type                                          | Description |
+| :------------------ | :-------------------------------------------- | :---------- |
+| **Cleared**         | `EventHandler`                                | Raised when all strokes are cleared. |
+| **StrokeCompleted** | `EventHandler<SKSignatureStrokeCompletedEventArgs>` | Raised when a stroke is completed. |
+
+## Commands
+
+| Command                       | Description |
+| :---------------------------- | :---------- |
+| **ClearedCommand**            | Executed when strokes are cleared. |
+| **ClearedCommandParameter**   | Parameter passed to the ClearedCommand. |
+| **StrokeCompletedCommand**    | Executed when a stroke is completed. |
+| **StrokeCompletedCommandParameter** | Parameter passed to the StrokeCompletedCommand. |
+
+## Parts
+
+The default template uses an SKCanvasView for rendering:
+
+```xaml
+<ControlTemplate x:Key="SKSignaturePadViewControlTemplate">
+    <skia:SKCanvasView x:Name="PART_DrawingSurface" />
+</ControlTemplate>
+```
+
+| Part                     | Description |
+| :----------------------- | :---------- |
+| **PART_DrawingSurface**  | This part can either be a `SKCanvasView` or a `SKGLView` and describes the actual rendering surface for the signature. |
+
+## Usage
+
+### Basic Usage
+
+```xaml
+<skia:SKSignaturePadView
+    x:Name="signaturePad"
+    HeightRequest="200"
+    StrokeColor="DarkBlue"
+    MinStrokeWidth="1"
+    MaxStrokeWidth="10"
+    PadBackgroundColor="Ivory" />
+```
+
+### With Commands
+
+```xaml
+<skia:SKSignaturePadView
+    x:Name="signaturePad"
+    HeightRequest="200"
+    StrokeCompletedCommand="{Binding OnStrokeCompleted}"
+    ClearedCommand="{Binding OnCleared}" />
+
+<Button Text="Clear" Command="{Binding ClearCommand}" />
+```
+
+### Exporting Signature
+
+```csharp
+// Get the signature as an image
+var image = signaturePad.ToImage(400, 200, SKColors.White);
+
+// Get the signature as a path for vector export
+var path = signaturePad.ToPath();
+
+// Check if signature exists
+if (!signaturePad.IsBlank)
+{
+    // Process signature
+}
+```
+
+### Undo Support
+
+```csharp
+// Undo the last stroke
+if (signaturePad.Undo())
+{
+    // Stroke was removed
+}
+```
+
+## Pressure Sensitivity
+
+The signature pad automatically uses pressure data from stylus/pen input when available:
+
+- **No pressure data (finger touch)**: Uses a default pressure of 0.5, resulting in medium stroke width
+- **Pressure data available**: Maps pressure (0.0-1.0) to stroke width between `MinStrokeWidth` and `MaxStrokeWidth`
+- **Harder pressure**: Produces thicker lines
+- **Lighter pressure**: Produces thinner lines
+
+## Ink Rendering Algorithm
+
+The signature pad uses a sophisticated algorithm for fluid ink rendering:
+
+1. **Point Collection**: Touch points are collected with associated pressure values
+2. **Minimum Distance Filtering**: Points too close together are filtered out to reduce noise
+3. **Quadratic Bezier Smoothing**: Points are interpolated using quadratic Bezier curves for smooth paths
+4. **Variable Width Calculation**: Stroke width is calculated at each point based on pressure
+5. **Polygon Generation**: The stroke is rendered as a filled polygon with offset curves on each side
+6. **Rounded Caps**: Start and end caps are added for natural ink appearance
+
+This approach mimics the fluid ink rendering found in Windows Ink and other professional digital ink systems.

docs/docs/toc.yml

@@ -15,4 +15,6 @@ items:
 - name: SKConfettiView
   href: skconfettiview.md
 - name: SKLottieView
-  href: sklottieview.md
+  href: sklottieview.md
+- name: SKSignaturePadView
+  href: sksignaturepadview.md