test: pin pixel-storybook to the partial-raster build

Not for merge. Vendors a tarball built from coder/pixel-storybook@cc953b4
(coder/pixel-storybook#57, --disable-partial-raster) so the Pixel job can
be re-run on this branch to check whether the 1/255 antialiasing jitter
seen in builds #26 to #28 is gone.

The tarball is vendored because coder/pixel-storybook is an internal repo
and CI checkouts have no credentials to fetch a git dependency.

Author: EhabY

.pixel-experiment/pixel-storybook-cc953b4.tgz

@@ -789,7 +789,7 @@
 		"zod": "^4.4.3"
 	},
 	"devDependencies": {
-		"@coder/pixel-storybook": "^0.2.2",
+		"@coder/pixel-storybook": "file:.pixel-experiment/pixel-storybook-cc953b4.tgz",
 		"@eslint-react/eslint-plugin": "^5.18.0",
 		"@eslint/js": "^10.0.1",
 		"@eslint/markdown": "^8.0.3",