[refactor] Rename FieldBuildingSection stories to match heading variants

`Default` previously used `headingVariant: 'compact'` while `DefaultHeading`
omitted the prop (giving the actual `'default'` variant). Swap so `Default`
maps to the default heading variant and `Compact` documents the compact one.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: joshestein

apps/website/src/components/lander/components/FieldBuildingSection.stories.tsx

@@ -60,15 +60,15 @@ export const Default: Story = {
   args: {
     title: 'Help build the field',
     intro: 'We also hire Adjunct Experts and Facilitators (~5h/week) and Fellow-Researchers (20-30h/week) to teach.',
-    headingVariant: 'compact',
     roles: sampleRoles,
   },
 };
 
-export const DefaultHeading: Story = {
+export const Compact: Story = {
   args: {
     title: 'Help build the field',
     intro: 'We also hire Adjunct Experts and Facilitators (~5h/week) and Fellow-Researchers (20-30h/week) to teach.',
+    headingVariant: 'compact',
     roles: sampleRoles,
   },
 };