test: add CodeBounty issue 79 markdown fixture

[TJCurnutte]

Summary

• Adds a deterministic CodeBounty issue test issue april 3 12pm #79 fixture capturing the live issue snapshot, markdown sample, and failed-upload marker.
• Adds a local validator that checks issue state, visible $50 bot-comment bounty source, required linkage, collision snapshot, and payout boundary.
• Documents the payout blocker clearly: developer_application_required and bounty_label_absent mean this is submitted-visible only, not verified-payable.

fixes #79

Validation

python3 -m py_compile scripts/validate-codebounty-issue-79.py
python3 scripts/validate-codebounty-issue-79.py test-fixtures/codebounty-issue-79-markdown-upload.json
git diff --check

Validator output:

{"issue": 79, "ok": true, "required_pr_linkage": "fixes #79", "same_scope_open_pr_count": 0, "verified_payable_at_snapshot": false, "visible_amount_source": "bot_comments", "visible_amount_usd": 50}

Collision / payout proof

• Live check before push: issue open, unlocked, unassigned, labels=enhancement, 2 CodeBounty bot comments.
• Repo check before push: public and not archived.
• Open PR collision terms checked immediately before push: 79, fixes #79, issue 79, test issue april 3 12pm.
• Same-scope open PR count before push: 0.
• Visible amount source: two CodeBounty bot comments announcing $50 USD.
• Payout boundary: platform application/maintainer acceptance is not verified; this PR must not be treated as paid or verified-payable until CodeBounty eligibility and acceptance are confirmed.