Fix MinZoom/MaxZoom having no effect on iOS

[Copilot]

MinZoom and MaxZoom were silently ignored on iOS. When FitPolicy.Width is active, PDFKit's AutoScales = true overrides MinScaleFactor/MaxScaleFactor for gesture-based zooming, making it impossible to constrain zoom bounds declaratively. Additionally, the old EnableZoom setter applied MinZoom/MaxZoom as raw absolute scale factors rather than relative to the fit scale, so the values were semantically wrong even when they did get applied.

Changes

• New ApplyZoomConstraints() method — sets MinScaleFactor = MinZoom × fitScale and MaxScaleFactor = MaxZoom × fitScale on the native PDFView, where fitScale is the scale factor computed by the active FitPolicy. This aligns iOS semantics with Android (where 1.0 = cannot zoom below fit, 3.0 = can zoom to 3× fit).

• Deferred application via _needsZoomConstraintsApply flag — ApplyFitPolicy() sets the flag; LayoutSubviewsAction processes it once the view has valid bounds and a stable ScaleFactor. This handles both the initial load (where AutoScales hasn't settled yet) and runtime FitPolicy changes.

• EnableZoom, MinZoom, MaxZoom are now reactive — each property stores a backing field and calls ApplyZoomConstraints() immediately (or defers if the document isn't loaded yet), so runtime updates propagate to the native layer.

<!-- MinZoom="1.0" now correctly prevents zooming out below fit-to-width on iOS -->
<pdf:PdfView EnableZoom="True"
             MinZoom="1.0"
             MaxZoom="3.0"
             FitPolicy="Width" />

The user-space workaround of setting nativePdfView.MinScaleFactor = nativePdfView.ScaleFactor in OnRendered is no longer needed.