Lightweight, parallel-safe progress counters for the corpus test layers.

Both corpus suites run one async test per entry, so work happens concurrently across schedulers. start/5 registers a shared, lock-free :atomics counter (in :persistent_term) under a caller-chosen key; tick/1 bumps it and prints a <verb> Q out of P <unit> line every step ticks (and on the last).

Each phase uses its own key (the over-firing analyze pass and the fix-safety pass run in the same mix test and would otherwise clobber one counter), so they report independently. tick/1 is a no-op when no tracker is registered for the key, keeping the same code paths silent under the mix credence.corpus task and the rule unit tests.