fix(mobile): tighten avatar shape guard and iOS agent fallback

The iOS Add members fallback tile drew agents as a 30% rounded rect while their loaded avatar was a squircle. It now draws the same squircle. The geometry test now rejects the old rounded rect, and the source guard also flags the old 30%/60% agent radius idiom.

Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>
Signed-off-by: Tom Brow <tomb@squareup.com>

mobile/ios/Runner/NativeAddMembersSheet.swift

@@ -281,8 +281,8 @@ final class NativeAddMembersViewController: UITableViewController, UISearchResul
     let size = CGSize(width: 40, height: 40)
     return UIGraphicsImageRenderer(size: size).image { _ in
       (Self.color(user["avatarBackground"]) ?? .tertiarySystemFill).setFill()
-      UIBezierPath(roundedRect: CGRect(origin: .zero, size: size),
+      let rect = CGRect(origin: .zero, size: size)
-        cornerRadius: user["agent"] as? Bool == true ? 12 : 20).fill()
+      (user["agent"] as? Bool == true ? Self.agentSquircle(rect) : UIBezierPath(ovalIn: rect)).fill()
       let text = (user["initial"] as? String ?? "?") as NSString
       let attributes: [NSAttributedString.Key: Any] = [
         .font: UIFont.systemFont(ofSize: 18, weight: .medium),
@@ -293,6 +293,21 @@ final class NativeAddMembersViewController: UITableViewController, UISearchResul
     }.withRenderingMode(.alwaysOriginal)
   }
 
+  /// Mirrors avatar_shape.dart and desktop's avatar-squircle.svg.
+  private static func agentSquircle(_ rect: CGRect) -> UIBezierPath {
+    func at(_ x: CGFloat, _ y: CGFloat) -> CGPoint {
+      CGPoint(x: rect.minX + rect.width * x, y: rect.minY + rect.height * y)
+    }
+    let path = UIBezierPath()
+    path.move(to: at(0.5, 0))
+    path.addCurve(to: at(1, 0.5), controlPoint1: at(0.93, 0), controlPoint2: at(1, 0.07))
+    path.addCurve(to: at(0.5, 1), controlPoint1: at(1, 0.93), controlPoint2: at(0.93, 1))
+    path.addCurve(to: at(0, 0.5), controlPoint1: at(0.07, 1), controlPoint2: at(0, 0.93))
+    path.addCurve(to: at(0.5, 0), controlPoint1: at(0, 0.07), controlPoint2: at(0.07, 0))
+    path.close()
+    return path
+  }
+
   private static func color(_ value: Any?) -> UIColor? {
     guard let number = value as? NSNumber else { return nil }
     let argb = number.uint32Value

mobile/test/shared/widgets/avatar_shape_test.dart

@@ -20,6 +20,9 @@ void main() {
       (const Offset(92, 8), false),
       (const Offset(92, 92), false),
       (const Offset(8, 92), false),
+      // Inside the old 30% rounded rect, outside the squircle.
+      (const Offset(1.9, 22), false),
+      (const Offset(98.1, 78), false),
     ]) {
       expect(path.contains(point), inside, reason: '$point');
     }
@@ -33,7 +36,9 @@ void main() {
   test('avatar shapes come only from avatarShape', () {
     // Agent avatars drifted into circles and 30% rounded rects because each
     // surface picked its own shape. Shape decisions keyed on agent identity,
-    // and person shapes on agent-only surfaces, must use avatar_shape.dart.
+    // the old agent radius idiom, and person shapes on agent-only surfaces
+    // must use avatar_shape.dart. Best effort: it matches the patterns that
+    // produced past drift, not every possible spelling.
     final inlineAgentShape = RegExp(
       r'\b(isAgent|isBot|agent)\b\s*\?\s*(const\s+)?'
       r'(BoxShape|BorderRadius|Radius|CircleBorder|RoundedRectangleBorder|'
@@ -43,6 +48,10 @@ void main() {
       r'if\s*\(\s*!?\s*(isAgent|isBot|agent)\s*\)\s*return\s+'
       r'(ClipOval|ClipRRect|CircleAvatar)\b',
     );
+    // The old hand-rolled agent radius: 30% of a size, or 60% of a radius.
+    final agentRadius = RegExp(
+      r'Radius\.circular\(\s*[\w.()+\s]*\*\s*0\.[36]\s*,?\s*\)',
+    );
     final personShape = RegExp(r'\b(ClipOval|CircleAvatar|BoxShape\.circle)\b');
     final violations = <String>[];
     for (final file in Directory('lib').listSync(recursive: true)) {
@@ -53,6 +62,7 @@ void main() {
       for (final match in [
         ...inlineAgentShape.allMatches(source),
         ...inlineAgentClip.allMatches(source),
+        ...agentRadius.allMatches(source),
         if (path.startsWith('lib/features/agents/'))
           ...personShape.allMatches(source),
       ]) {