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/SVGPathParser.swift

@@ -169,7 +169,7 @@ private struct Tokenizer {
 
   mutating func readNumber() -> CGFloat {
     skipSeparators()
-    var start = index
+    let start = index
     var sawDot = false
     var sawE = false
     while index < chars.count {