Add OnCameraPreviewReady event

[VagnerVit]

Summary

Adds a new OnCameraPreviewReady event and OnCameraPreviewReadyCommand bindable property to CameraView, fired once when the camera preview becomes ready.

Closes #194

Motivation

There is currently no public API to know when the camera preview is actually rendering. Users resort to fragile workarounds like fixed Task.Delay or monitoring CurrentZoomFactor via PropertyChanged to hide loading overlays at the right moment.

Changes

• New OnCameraPreviewReadyEventArg — simple EventArgs subclass (follows existing pattern)
• CameraView — new event, bindable command, and internal trigger method with a guard flag ensuring it fires only once per camera session (reset on Start)
• Android — fires in CameraStateObserver.OnChanged() when CameraState.Type.Open is reached
• iOS/macOS — fires at the end of the Start() dispatch block after session starts running

Usage

<scanner:CameraView OnCameraPreviewReady="OnCameraPreviewReady" />

// Event
cameraView.OnCameraPreviewReady += (s, e) => { /* hide overlay */ };

// Or via command binding
OnCameraPreviewReadyCommand="{Binding CameraReadyCommand}"