Implement Picker control

[Copilot]

Adds dropdown/selection control using native platform widgets: UIPickerView (iOS) and Spinner (Android).

Implementation

Core (Core/Picker.cs)

• Items: ObservableCollection<string> - data source
• SelectedIndex: int - current selection (-1 for none)
• SelectedItem: string? - computed from index, null-safe
• Title: string - prompt text (Android only)
• TextColor: Color? - item text color

iOS (Platforms/iOS/Picker.cs)

• Custom UIPickerViewModel with WeakReference<Picker> to prevent retain cycles
• Single-component picker with dynamic row count from Items collection

Android (Platforms/Android/Picker.cs)

• ArrayAdapter<string> backed by Items collection
• Circular event prevention: updates only when e.Position != _selectedIndex

Usage

var picker = new Picker
{
    Title = "Select an option",
    Items = new ObservableCollection<string> { "Red", "Green", "Blue" }
};

picker.PropertyChanged += (s, e) => 
{
    if (e.PropertyName == nameof(picker.SelectedIndex))
        Debug.WriteLine($"Selected: {picker.SelectedItem}");
};

Tests & Sample

• 9 unit tests covering property notifications, collection ops, null safety
• PickerScenario demonstrates color selection with live UI updates
• Updated comparison doc: 6→7 implemented controls

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Pull request overview

Adds a new cross-platform Picker control to Spice (core + iOS + Android), along with unit tests, a scenario sample, and documentation updates reflecting the new control.

Changes:

• Introduces Picker core API (Items, SelectedIndex, SelectedItem, Title, TextColor).
• Adds native implementations using UIPickerView (iOS) and Spinner (Android).
• Adds unit tests + a scenario demo, and updates the MAUI controls comparison doc.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/Spice.Tests/PickerTests.cs	Adds unit tests for Picker core behaviors and property notifications.
src/Spice/Core/Picker.cs	Defines the core Picker API and selection model.
src/Spice/Platforms/iOS/Picker.cs	Implements iOS native wiring via UIPickerView + model.
src/Spice/Platforms/Android/Picker.cs	Implements Android native wiring via Spinner + ArrayAdapter.
samples/Spice.Scenarios/Scenarios/PickerScenario.cs	Adds a scenario demonstrating picker-driven color selection.
samples/Spice.Scenarios/App.cs	Adds a navigation button to launch the new picker scenario.
docs/MAUI-CONTROLS-COMPARISON.md	Marks Picker as implemented and updates counts/mappings.