Greptile P1: the retention test mutated the module-level `LAST_FAILURE` and
left it set on exit. `cargo test` runs a binary's tests in parallel, so any
future test asserting on `last_failure_message()` would race it.
Rather than paper over it with teardown (which doesn't fix the race, only the
leak), `set_stage` now delegates to `set_stage_into(state, slot, stage)` with
the retention slot as a parameter. The behavioural tests drive their own slot —
deterministic and parallel-safe by construction — plus one narrow wiring test
that asserts the public `set_stage` writes the same global `last_failure_message`
reads, using a thread-unique value so a parallel writer cannot make it flake.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
