fix(pen-core): nav fill inject preserves gradient / image fills

Previous `hasSolidFill` only matched `type === 'solid'`. Sub-agents
legitimately put `linear_gradient` (sunrise hero, accent ribbon),
`radial_gradient` (splash entries), or `image` (branded photo banners)
on top app bars and other nav surfaces, and `hasSolidFill` would
return false for those — making the inject pass overwrite the
gradient/image with a flat `$color-surface` solid.

Renamed to `hasAnyFill`; matches any first-entry shape with a
recognized `type` field. Sub-agent intent (any non-empty fill) now
short-circuits the inject. Three new tests cover linear gradient,
radial gradient, and image fills explicitly — all preserved.

packages/pen-core/src/__tests__/inject-nav-surface-fill.test.ts

@@ -50,6 +50,82 @@ describe('injectMissingNavSurfaceFill', () => {
     expect((navbar as PenNode & { fill?: unknown }).fill).toEqual(solidFill('#0F172A'));
   });
 
+  it('preserves a linear-gradient nav fill (does not overwrite with solid)', () => {
+    const gradientFill = [
+      {
+        type: 'linear_gradient' as const,
+        stops: [
+          { offset: 0, color: '#F97316' },
+          { offset: 1, color: '#EA580C' },
+        ],
+        angle: 90,
+      },
+    ];
+    const navbar = frame({
+      id: 'top-bar',
+      role: 'top-app-bar',
+      fill: gradientFill as never,
+      children: [],
+    });
+    const root = frame({
+      id: 'root',
+      fill: solidFill('#FFFFFF'),
+      children: [navbar],
+    });
+    const changed = injectMissingNavSurfaceFill(root);
+    expect(changed).toBe(false);
+    expect((navbar as PenNode & { fill?: unknown }).fill).toEqual(gradientFill);
+  });
+
+  it('preserves a radial-gradient nav fill', () => {
+    const radial = [
+      {
+        type: 'radial_gradient' as const,
+        stops: [
+          { offset: 0, color: '#FFFFFF' },
+          { offset: 1, color: '#000000' },
+        ],
+        center: { x: 0.5, y: 0.5 },
+      },
+    ];
+    const navbar = frame({
+      id: 'splash-nav',
+      role: 'navbar',
+      fill: radial as never,
+      children: [],
+    });
+    const root = frame({
+      id: 'root',
+      fill: solidFill('#FFF8F0'),
+      children: [navbar],
+    });
+    injectMissingNavSurfaceFill(root);
+    expect((navbar as PenNode & { fill?: unknown }).fill).toEqual(radial);
+  });
+
+  it('preserves an image nav fill (branded nav surface with photo)', () => {
+    const imageFill = [
+      {
+        type: 'image' as const,
+        src: 'https://example.com/banner.jpg',
+        scaleMode: 'cover',
+      },
+    ];
+    const navbar = frame({
+      id: 'hero-nav',
+      role: 'top-app-bar',
+      fill: imageFill as never,
+      children: [],
+    });
+    const root = frame({
+      id: 'root',
+      fill: solidFill('#FFFFFF'),
+      children: [navbar],
+    });
+    injectMissingNavSurfaceFill(root);
+    expect((navbar as PenNode & { fill?: unknown }).fill).toEqual(imageFill);
+  });
+
   it('does not touch nav frames nested inside cards/sections', () => {
     const nestedNav = frame({
       id: 'inner-nav',

packages/pen-core/src/layout/inject-nav-surface-fill.ts

@@ -39,7 +39,7 @@ export function injectMissingNavSurfaceFill(rootFrame: PenNode): boolean {
     const role = (child as PenNode & { role?: string }).role;
     if (!role || !NAV_ROLES.has(role)) continue;
     const existing = (child as PenNode & { fill?: PenFill[] | string }).fill;
-    if (hasSolidFill(existing)) continue;
+    if (hasAnyFill(existing)) continue;
     (child as PenNode & { fill?: PenFill[] }).fill = [
       { type: 'solid', color: '$color-surface' } as SolidFill,
     ];
@@ -48,10 +48,19 @@ export function injectMissingNavSurfaceFill(rootFrame: PenNode): boolean {
   return changed;
 }
 
-function hasSolidFill(fill: PenFill[] | string | undefined): boolean {
+/**
+ * True when the frame already carries ANY valid fill the renderer would
+ * paint — solid OR gradient (linear/radial) OR image. We must NOT
+ * overwrite a non-solid fill with our default `$color-surface` solid:
+ * sub-agents legitimately put `linear_gradient` on top app bars
+ * (sunrise hero gradient), `image` on hero / branded nav surfaces, and
+ * `radial_gradient` on splash-style entries. Keep their intent.
+ */
+function hasAnyFill(fill: PenFill[] | string | undefined): boolean {
   if (!fill) return false;
   if (typeof fill === 'string') return fill.length > 0;
   if (!Array.isArray(fill) || fill.length === 0) return false;
+  // Any first entry with a recognized `type` counts as intentional fill.
   const first = fill[0];
-  return first?.type === 'solid' && typeof (first as SolidFill).color === 'string';
+  return !!first && typeof (first as { type?: string }).type === 'string';
 }