Pin Arena environments to stable Verifiers

docs/PRIME_FIRST.md

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+# Prime-first boundary
+
+Arena publishes Lumbridge environments; it does not implement another generic
+harness, trainer, runtime marketplace, or environment hub.
+
+- Every environment pins stable `verifiers==0.3.1`.
+- Pure-trace environments select Verifiers' upstream `null` harness.
+- Local model-free and spark-1 runs name the `subprocess` runtime explicitly so
+  tests never provision a paid Prime sandbox.
+- Prime Hub is the public distribution target for approved environment wheels.
+- Prime-RL consumes the same taskset packages for training; Forge owns the
+  explicit training handoff and artifact rendering.
+
+Lumbridge's unique work is the environment: task distributions, interactive
+state, rewards, probes, held-out gates, Tera bridges, and result interpretation.
+Generic rollout and training machinery stays upstream. A Hub publication must
+exclude secrets, private task splits, customer data, and proprietary Tera source.
+
+The preferred future compute path is Prime's spot market. GH200/GB300 names in
+runbooks are desired hardware profiles, not a promise that inventory exists and
+not authorization to rent it. An operator checks current Prime availability and
+price before launching any paid run.