Read back the coverage the metamutant recorded during the probe run.

The metamutant self-records coverage synchronously, in the test process, as the suite runs (Mutare.Coverage.Recorder owns the generated side); at suite end an ExUnit.after_suite/1 hook dumps it to a file. This module reads that dump.

The dump is three things, all keyed by mutant id (no metamutant↔original line mapping — the original line is only for the report):

• :aggregate — the set of mutant ids whose selector ran at all, in any process (test body, setup, setup_all, a spawned task). This is the no-coverage signal: an id not in it can never be killed, so skip it and keep it out of the score's denominator.
• :by_file — %{test_file => MapSet(mutant ids)}: which mutant ids each test file covered. A line running in the test process is labeled directly; one running in a Task it spawned is attributed via the caller chain; one running in a module's setup_all is attributed via the __ex_unit__/2 stacktrace frame (module-granular, the file selection needs). This drives per-file test selection.
• :unlabeled — the set of mutant ids whose selector ran in a process with no recoverable test label at all — a bare spawn, a setup-registered on_exit closure, or the rare setup_all whose work happened off-stack in a Task it spawned (an on_exit registered in a test body is recovered, via its closure frame in ExUnit's per-test runner process). An id here was covered, but which test owns it is unknown, so the caller runs the whole suite for it — even if :by_file also attributes it to some file, since that partial attribution would otherwise mask the unlabeled coverage and manufacture a false survivor.

Two more keys carry the finer test-case granularity :tests selection uses (both keyed by mutant id, both empty in a dump written before this contract, so an old dump degrades :tests to :coverage):

• :by_test — %{mutant id => MapSet(runnable test names)}: the individual ExUnit tests (test/doctest/property names) that covered each id, so a mutant can run mix test <file> --only test:<name> instead of the whole file.
• :wholefile — the set of ids with a labeled but non-narrowable attribution (a setup_all or an on_exit, which cover through a module-scoped context, not a single runnable test). :tests must run the whole file for such an id — narrowing to named tests would drop the covering context and manufacture a false survivor.

Mutare.Runner.CoverageProbe reconciles them all.

Why not :cover: its counters live in a single global table keyed {module, line} with no per-process partition, so attributing coverage to a test in one run needs a per-test snapshot, and the only global per-test signal ExUnit emits is an async cast to formatters that races test execution (fast async: false modules' coverage is lost). Self-recording in the metamutant sidesteps that entirely — and is process-agnostic for the aggregate, so it is a better no-coverage detector than :cover ever was.