feat(1.1): supervise cpu group with ProcessPoolExecutor kernel offload

Hosts clusterer/campaign-clusterer/attribution/reuse-correlate in one
process. The two O(n^2) connected-components kernels (cluster_observations,
cluster_identities) offload to ONE shared forkserver pool via decnet.offload
.run_kernel, so they run in parallel instead of serialising under the GIL.

- offload.run_kernel: pool when installed + offload_if holds, else inline.
  Standalone workers and all tests run inline => behaviour unchanged
  (424 clustering/correlation tests green).
- offload_if gates on input size (>=256) to skip pickle cost on small passes.
- forkserver (not fork): supervisor is multithreaded via bus clients.
- attribution/reuse co-located but not offloaded yet (lighter; same run_kernel
  path extends to them if profiling shows contention).
- systemd unit Conflicts= the 4 units it replaces; no docker/raw-socket priv.

tests/test_offload.py

@@ -0,0 +1,68 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Tests for the shared CPU-kernel offload (DECNET 1.1 cpu group).
+
+Proves the offloaded result is identical to the inline result — i.e. the kernel
+and its inputs survive the process boundary and the GIL-relief path is correct,
+not just fast.
+"""
+from __future__ import annotations
+
+import multiprocessing as mp
+from concurrent.futures import ProcessPoolExecutor
+
+import pytest
+
+from decnet import offload
+from decnet.clustering.impl.connected_components import cluster_observations
+from decnet.clustering.impl.similarity import Observation
+
+pytestmark = pytest.mark.asyncio
+
+
+@pytest.fixture(autouse=True)
+def _clear_executor():
+    offload.set_executor(None)
+    yield
+    offload.set_executor(None)
+
+
+async def test_inline_when_no_executor():
+    assert offload.get_executor() is None
+    out = await offload.run_kernel(lambda a, b: a + b, 2, 3)
+    assert out == 5  # closures are fine on the inline path (no pickling)
+
+
+async def test_offload_if_false_runs_inline_even_with_pool():
+    with ProcessPoolExecutor(
+        max_workers=1, mp_context=mp.get_context("forkserver")
+    ) as pool:
+        offload.set_executor(pool)
+        # a closure would fail to pickle — proves this stayed inline
+        out = await offload.run_kernel(lambda x: x * 10, 4, offload_if=False)
+        assert out == 40
+
+
+async def test_offloaded_result_equals_inline():
+    obs = [
+        Observation(observation_id="a", ja3="x", hassh=None, asn=1),
+        Observation(observation_id="b", ja3="x", hassh=None, asn=1),
+        Observation(observation_id="c", ja3="y", hassh=None, asn=2),
+    ]
+    inline = cluster_observations(obs)
+
+    with ProcessPoolExecutor(
+        max_workers=2, mp_context=mp.get_context("forkserver")
+    ) as pool:
+        offload.set_executor(pool)
+        offloaded = await offload.run_kernel(cluster_observations, obs)
+
+    assert offloaded == inline  # identical across the process boundary
+
+
+async def test_set_get_executor_roundtrip():
+    assert offload.get_executor() is None
+    with ProcessPoolExecutor(max_workers=1) as pool:
+        offload.set_executor(pool)
+        assert offload.get_executor() is pool
+    offload.set_executor(None)
+    assert offload.get_executor() is None