The matrix had three serial stretches left. `run_compat.sh` only ever
handed six of the sixteen feature directories to `pytest -n`, so the
other ten columns ran one cell at a time; the ten HTTP-probe cells
looped their three Claude tiers in sequence, paying the per-provider
rate limiter's wait three times per cell; and the session-scoped
fixture that registers the fifteen compat deployments POSTed them one
at a time, in every xdist worker process
Registration moves to `pytest_configure` on the controller and goes out
in parallel, so a run registers each deployment once instead of once
per worker. Workers learn the controller owns registration through
`pytest_configure_node`; a run whose controller never loaded this
conftest still registers per worker, so no invocation loses its
deployments. The probe cells share one `run_probe_cell` body that fans
the tiers out and reports rows in declared order, replacing ten copies
of the same loop
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