test: derive missing-subject squash fixture

[200lz]

Summary

• derive the historical authoritative-main squash commit that actually requires live public corroboration
• preserve offline (#N) recognition for the current and earlier ordinary squash commits
• route mocked GitHub responses to exact endpoints without corrupting unrelated current-PR evidence
• keep the correction entirely test-only

Root cause

After PR #6 merged with the normal (#6) subject, current main correctly used OFFLINE_SUBJECT_SUFFIX.

The integration-style test fixture still generated mocked live metadata from current origin/main, while the production audit requested live corroboration for the historical PR #5 squash commit whose custom subject omitted (#5). That mismatch failed closed as ASSOCIATED_PR_RESULT_SHA_MISMATCH.

Scope

Only tests/test_public_release_readiness.py is changed. Production audit logic, workflows, documentation, README, case-study evidence, schemas, and dependencies are unchanged.

Validation

• two previously failing focused tests passed
• readiness module: 72 passed
• complete offline suite: 1,897 passed, 9 expected skips
• compilation, Ruff, formatting, mypy, JSON parsing, and git diff --check passed
• Unsloth case-study checksums and bytes remained unchanged