ci: enforce minimum package coverage

[stefan-jansen]

Summary

• set the package coverage floor to 85%, below the fresh 85.98% baseline
• add a dedicated pull-request coverage job
• enforce the same coverage lane during release qualification
• make the README verification command match CI

Verification

• focused release tests fail at 14.54%, proving the threshold can fail
• full offline suite passes at 85.98% with 3,759 tests
• Ruff, formatting, ty, actionlint, and diff checks pass

Closes #79

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