test: add CodeBounty issue 10 fixture

[TJCurnutte]

Summary

• Adds a deterministic fixture for CodeBounty issue Testing new UI changes #10's UI-layout bounty flow.
• Records the public $30 CodeBounty bot announcement, bounty label, required Fixes #10 linkage, collision snapshot, and payout boundary.
• Adds a stdlib validator plus a short validation note so the fixture can be checked without CodeBounty platform credentials.

Fixes #10

Validation

python3 -m py_compile scripts/validate-codebounty-issue-10.py
python3 scripts/validate-codebounty-issue-10.py test-fixtures/codebounty-issue-10-ui-layout.json
git diff --check

Collision / payout proof

Fresh pre-push checks on 2026-05-14 UTC:

• Issue Testing new UI changes #10 was open, unlocked, unassigned, and labeled bug + 💰 Bounty Available.
• Repo CodeBountyOrg/BountyTestRepository was public and not archived.
• 37 open PRs were checked.
• Search terms: 10, #10, fixes #10, issue 10, Testing new UI changes, new UI changes.
• Raw search hits: 4; exact same-scope open PR collisions after filtering: 0.
• Visible amount source: public CodeBounty bot comment ($30), with platform application/eligibility still required.

Submitted-visible only. Verified-payable remains $0 until CodeBounty/maintainer acceptance and payout eligibility are verified.