A frame with borders tiles unless told to stretch

The fill of a sliced frame defaulted to `Stretch`. It is `Repeat` now, and the Fill dropdown lists
`Repeat` first.

The saved document keeps writing only the fill that differs from the default, so the key it
carries is now `stretch`, and a frame without it repeats. A frame saved before with the key
absent and meaning stretch would therefore tile; none of the stored projects has one - both sliced
frames on the server carry `fill: "repeat"` explicitly, so they draw as before.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/core/serialize.test.ts

@@ -75,7 +75,7 @@ describe('serialize', () => {
               w: 24,
               h: 24,
               border: { left: 8, right: 8 },
-              fill: 'repeat',
+              fill: 'stretch',
             },
             { id: 'f2', name: 'Plain', x: 24, y: 0, w: 24, h: 24 },
             {
@@ -96,7 +96,8 @@ describe('serialize', () => {
     if (texture.type !== 'texture') throw new Error('wrong types');
     const [sliced, plain, greedy] = texture.frames;
     expect(sliced?.border).toEqual({ left: 8, top: 0, right: 8, bottom: 0 });
-    expect(sliced?.fill).toBe('repeat');
+    // Only the fill that is not the default is written; an unknown one reads as the default.
+    expect(sliced?.fill).toBe('stretch');
     expect(plain?.border).toBeUndefined();
     expect(plain?.fill).toBeUndefined();
     // Measured against the frame, not the image, and never leaving it without a middle.