test: add CodeBounty issue 17 fixture

[TJCurnutte]

Summary

• Adds a deterministic JSON fixture for CodeBounty issue codebountry #17 (codebountry) with the public issue URL, $120 visible bounty amount, announcement URL, and required fixes #17 PR phrase.
• Adds a dependency-free validator that checks the fixture amount, issue number, normalized issue key, PR phrase, and submitted-visible vs verified-payable payout boundary.
• Adds a short receipt note documenting scope and validation.

Fixes #17

Validation

• python3 -m py_compile scripts/validate-codebounty-issue-17.py
• python3 scripts/validate-codebounty-issue-17.py test-fixtures/codebounty-issue-17.json
• Python fixture assertions for issue number, $120 amount, fixes #17, and verified-payable boundary
• git diff --check

Fresh collision / state check

• Re-fetched issue codebountry #17 before push: state OPEN, label 💰 Bounty Available, no assignees, max visible amount $120.
• Re-fetched 27 open PRs immediately before push; exact search for #17, fixes #17, issue-17 branch patterns, and codebountry found 0 same-scope open PR collisions.

Payout boundary

Submitted-visible only. CodeBounty platform application/eligibility and maintainer/platform acceptance are not verified by this PR.