Add undo/erase functionality with stroke tracking

[Copilot]

This PR implements undo/erase functionality for the Android signature pad by tracking user strokes and providing the ability to remove the last stroke.

Implementation Details

Core Changes

SignatureSDK (signature-core)

• Added strokeEndIndices to track stroke boundaries (complete DOWN→MOVE→UP sequences)
• Implemented canUndo(): Boolean to check if undo operations are available
• Implemented undo() to remove the last stroke and redraw the signature
• Updated clear() and restoreEvents() to properly handle stroke tracking

SignaturePad (signature-view)

• Exposed canUndo() and undo() methods that delegate to SignatureSDK
• Maintains existing API compatibility

SignaturePadAdapter (signature-pad)

• Added Compose-compatible canUndo() and undo() methods
• Enables undo functionality in Jetpack Compose applications

Usage Example

// Check if undo is available
if (signaturePad.canUndo()) {
    // Remove the last stroke
    signaturePad.undo()
}

// In Compose
SignaturePadView(
    onReady = { adapter ->
        // Button state management
        undoButton.isEnabled = adapter.canUndo()
        
        // Undo last stroke
        undoButton.setOnClickListener {
            adapter.undo()
        }
    }
)

Technical Approach

The implementation leverages the existing event system for minimal invasive changes:

1. Stroke Tracking: Records indices where strokes end (ACTION_UP events)
2. Event Removal: Removes complete strokes from the event list during undo
3. Efficient Redraw: Uses existing restoreEvents() method to redraw from remaining events
4. State Management: Provides canUndo() for proper UI control states

Sample Integration

The sample app has been updated with an undo button demonstrating the functionality:

• Button is enabled/disabled based on canUndo() state
• Properly updates other UI elements after undo operations
• Shows real-time undo capability alongside existing clear functionality

Validation

• ✅ All modules build successfully with updated APIs
• ✅ Stroke boundary logic validated with simulation testing
• ✅ Event management and redraw functionality verified
• ✅ Sample app demonstrates end-to-end undo capability
• ✅ Backward compatibility maintained

Fixes #354.

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