Add SelectPanel outside-top autogrow regression fixture and VRT test

[siddharthkp]

Closes #

This PR adds a fixture story and VRT test to reproduce and guard against a visual regression where SelectPanel with anchorSide: 'outside-top' and async-loading content briefly flickers downward before repositioning upward.

The bug:
When content loads asynchronously inside an outside-top anchored SelectPanel:

1. Content loads → panel height increases naturally (downward)
2. Position recalculates → panel jumps to correct outside-top position
3. This causes a visible flicker

This PR:

• Adds AutogrowAfterLoadingWithOutsideTopAnchor story to reproduce the issue
• Adds a VRT test to catch regressions
• Does not include a fix yet — this is a regression fixture only

Note: With CSS anchor positioning (primer_react_css_anchor_positioning flag enabled), the browser handles positioning natively and bottom: anchor(top) causes content to grow upward naturally — no flicker. The bug only affects the JS positioning fallback path.

Changelog

New

• SelectPanel.examples.stories.tsx: Added AutogrowAfterLoadingWithOutsideTopAnchor story

Changed

• None

Removed

• None

Rollout strategy

• None; this is a fixture/test-only change with no runtime impact

Testing & Reviewing

1. Open Storybook → SelectPanel → Examples → Autogrow After Loading With Outside Top Anchor
2. Click the button to open the panel
3. Observe the brief downward flicker before the panel repositions upward
4. With primer_react_css_anchor_positioning: true, the flicker should not occur

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

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[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.