fix(pen-core): preserve cornerRadius on media-clipping frames

Codex stop-hook on the prior strip-nested-card-decoration commit
caught a regression: cornerRadius on a media-clipping frame
(`clipContent: true` wrapping an image / video, or roles like
`image-placeholder` / `thumbnail` / `cover-image`) is doing the
rounding work for the photo, not stacking card decoration. Blanket
stripping un-rounded the media against the user's clear intent —
typical pattern is

  card { cornerRadius: 16, clipContent: true }
   └─ image-placeholder { cornerRadius: 12, clipContent: true }
       └─ image

where the inner cornerRadius rounds the photo and the outer rounds
the card frame around it. After the prior pass the inner radius got
stripped (ancestor had cornerRadius too) → square corners on the
photo.

New `MEDIA_CLIP_ROLES` set + `isMediaClipper(node)` helper:
  - role match: image, image-card, image-placeholder, video,
    video-placeholder, media, media-thumbnail, thumbnail, cover,
    cover-image, gallery-item
  - shape match: clipContent: true AND has a direct image / video /
    media-roled child

Either signal preserves cornerRadius. Other decorations (stroke,
shadow) still get stripped — those ARE redundant card decoration
even on a media wrapper, since the photo's own outline + the
ancestor card already provide the visual frame.

Tests: 2 new cases — clipContent + image, and the role-only path
covering image-placeholder / thumbnail / cover-image / gallery-item.

packages/pen-core/src/__tests__/strip-nested-card-decoration.test.ts

@@ -189,6 +189,62 @@ describe('stripNestedCardDecoration', () => {
     expect((inner as PenNode & { cornerRadius?: unknown }).cornerRadius).toBeUndefined();
   });
 
+  // 2026-05-11 Codex stop-hook caught: cornerRadius on a media-clipping
+  // frame (clipContent: true wrapping an image / video) is doing the
+  // rounding work, not stacking card decoration. Stripping it would
+  // un-round the photo against the user's intent.
+
+  it('preserves cornerRadius on a clipContent frame wrapping an image', () => {
+    const image = frame({ id: 'img', type: 'image' as const } as never);
+    const clipper = frame({
+      id: 'clipper',
+      cornerRadius: 12,
+      clipContent: true,
+      stroke: stroke(),
+      effects: shadow(),
+      children: [image],
+    } as never);
+    const card = frame({
+      id: 'card',
+      role: 'card',
+      cornerRadius: 16,
+      stroke: stroke(),
+      effects: shadow(),
+      children: [clipper],
+    } as never);
+
+    stripNestedCardDecoration(card);
+
+    // cornerRadius preserved (media clip role).
+    expect((clipper as PenNode & { cornerRadius?: unknown }).cornerRadius).toBe(12);
+    // stroke + shadow still get stripped (those are pure decoration).
+    expect((clipper as PenNode & { stroke?: unknown }).stroke).toBeUndefined();
+    expect((clipper as PenNode & { effects?: unknown }).effects).toBeUndefined();
+  });
+
+  it('preserves cornerRadius on frames with media role (image-placeholder, thumbnail, cover)', () => {
+    for (const role of ['image-placeholder', 'thumbnail', 'cover-image', 'gallery-item']) {
+      const inner = frame({
+        id: `inner-${role}`,
+        role,
+        cornerRadius: 8,
+      } as never);
+      const card = frame({
+        id: 'card',
+        role: 'card',
+        cornerRadius: 16,
+        children: [inner],
+      } as never);
+
+      stripNestedCardDecoration(card);
+
+      expect(
+        (inner as PenNode & { cornerRadius?: unknown }).cornerRadius,
+        `${role} should retain cornerRadius`,
+      ).toBe(8);
+    }
+  });
+
   it('returns false when nothing was modified', () => {
     const card = frame({
       id: 'plain-card',

packages/pen-core/src/layout/strip-nested-card-decoration.ts

@@ -56,6 +56,30 @@ const KEEP_DECORATION_ROLES = new Set([
   'segmented-control',
 ]);
 
+/**
+ * Roles that semantically carry MEDIA — their cornerRadius is doing
+ * clipping work (rounded photo / video / thumbnail) rather than
+ * card-style decoration. Stripping the radius would un-round the
+ * media against the user's clear intent. Codex 2026-05-11 stop-hook
+ * caught the regression: a card with cornerRadius wrapping an
+ * image-placeholder would have the placeholder's own cornerRadius
+ * (used for `clipContent: true` rounding) stripped → square corners
+ * on the photo even though the surrounding card was rounded.
+ */
+const MEDIA_CLIP_ROLES = new Set([
+  'image',
+  'image-card',
+  'image-placeholder',
+  'video',
+  'video-placeholder',
+  'media',
+  'media-thumbnail',
+  'thumbnail',
+  'cover',
+  'cover-image',
+  'gallery-item',
+]);
+
 interface DecoFlags {
   hasStroke: boolean;
   hasCornerRadius: boolean;
@@ -82,6 +106,26 @@ function isRoleProtected(node: PenNode): boolean {
   return KEEP_DECORATION_ROLES.has(role);
 }
 
+/**
+ * Frames acting as media-clipping containers — `clipContent: true` plus
+ * a direct image / video child mean the cornerRadius is rounding the
+ * photo, not stacking card decoration. Detector preserves cornerRadius
+ * on these (other decorations still get stripped if redundant).
+ */
+function isMediaClipper(node: PenNode): boolean {
+  const role = ((node as { role?: string }).role ?? '').toLowerCase();
+  if (MEDIA_CLIP_ROLES.has(role)) return true;
+  const clipContent = (node as PenNode & { clipContent?: unknown }).clipContent;
+  if (clipContent !== true) return false;
+  if (!('children' in node) || !Array.isArray(node.children)) return false;
+  return node.children.some((c) => {
+    if (!c) return false;
+    if (c.type === 'image') return true;
+    const childRole = ((c as { role?: string }).role ?? '').toLowerCase();
+    return MEDIA_CLIP_ROLES.has(childRole);
+  });
+}
+
 /**
  * Returns true if any node was modified.
  */
@@ -105,6 +149,7 @@ export function stripNestedCardDecoration(root: PenNode): boolean {
         );
       const own = readDecoration(node);
       const isProtected = isRoleProtected(node);
+      const mediaClipper = isMediaClipper(node);
       if (!isProtected) {
         const n = node as PenNode & {
           stroke?: unknown;
@@ -115,7 +160,10 @@ export function stripNestedCardDecoration(root: PenNode): boolean {
           delete n.stroke;
           changed = true;
         }
-        if (own.hasCornerRadius && ancestorDeco.hasCornerRadius) {
+        // Preserve cornerRadius on media-clipping frames — the radius is
+        // rounding the photo, not stacking card decoration. Other
+        // decorations (stroke / shadow) still get stripped if redundant.
+        if (own.hasCornerRadius && ancestorDeco.hasCornerRadius && !mediaClipper) {
           delete n.cornerRadius;
           changed = true;
         }