Rename native borderRadius prop to avoid UIView collision

RN/Expo already owns a borderRadius property on UIView (as a style prop),
so declaring `private var borderRadius` on ExpoView was rejected by Swift
("cannot override with a stored property"). Rename the bridge prop and Swift
property to `previewCornerRadius`; the public Trigger/ImageContextMenu API
keeps `borderRadius` and Root translates it at the bridge layer.

Also tighten `var start` to `let` in the SVG parser.

https://claude.ai/code/session_015REmux3R9uuEMMJUHxTyQT

modules/expo-bluesky-context-menu/ios/ExpoBlueskyContextMenuModule.swift

@@ -15,8 +15,9 @@ public class ExpoBlueskyContextMenuModule: Module {
         view.setMenuItems(value)
       }
 
-      Prop("borderRadius") { (view: ExpoBlueskyContextMenuView, value: Double) in
-        view.setBorderRadius(value)
+      Prop("previewCornerRadius") {
+        (view: ExpoBlueskyContextMenuView, value: Double) in
+        view.setPreviewCornerRadius(value)
       }
     }
   }