fix(scripts): exempt Storybook stories from coverage:diff gate

Story files are excluded from vitest by design (they run in Storybook
runner, not vitest). Add *.stories.{ts,tsx} to ALLOWED_GLOBS so the
L1 diff gate doesn't flag them as "new untested file".

Also add error-handling test for useOptionalConsent rethrow path
(cookie-consent-banner lines 52-53) achieving 100% statement coverage.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/coverage/diff.mjs

@@ -32,6 +32,8 @@ import { execSync } from "node:child_process";
 const ALLOWED_GLOBS = [
   // Test artifacts
   /\.test\.(ts|tsx|js|mjs)$/,
+  // Storybook story files — excluded from vitest by design; tested in Storybook runner
+  /\.stories\.(ts|tsx)$/,
   /\/__factories__\//,
   /\/__contracts__\//,
   /\/__fixtures__\//,