Incremental line-coverage gating for stale ExUnit runs.
mix test --cover only measures the tests that actually ran, so combining
it with mix test --stale reports partial numbers. qlover closes the gap:
it records a baseline from the last green full run and lets a later stale
run satisfy the coverage gate for everything the stale subset did not need
to re-execute. See Mix.Tasks.Qlover for the mechanism and README.md
for the background.