test: add CodeBounty issue 100 fixture

[TJCurnutte]

Summary

Fixes #100.

Adds a deterministic CodeBounty fixture and validator for issue #100's duplicate bounty-announcement snapshot.

Changes

• adds test-fixtures/codebounty-issue-100-duplicate-announcement.json
• adds scripts/validate-codebounty-issue-100.py
• adds bounty-notes/issue-100-duplicate-announcement.md

Validation

• python3 -m py_compile scripts/validate-codebounty-issue-100.py
• python3 scripts/validate-codebounty-issue-100.py test-fixtures/codebounty-issue-100-duplicate-announcement.json
• git diff --check

Collision check

Immediately before pushing this branch:

• repo: public and not archived
• issue issue apr 17 235 #100: open, unlocked, unassigned
• labels: bug, 💰 Bounty Available
• comments checked: 3
• open PR searches: 100, fixes #100, issue 100, apr 17 235
• exact same-scope open PR count: 0
• raw GitHub search hits: 3 false positives from $100 / issue #103 contexts, not issue apr 17 235 #100 linkage

Bounty / payout boundary

• CodeBounty visible amount: $50
• Required linkage: fixes #100
• developer_application_required: true
• duplicate_bot_announcements: true
• verified_payable=false until CodeBounty/maintainer acceptance and platform eligibility are confirmed.