fix: gate embedded sniffer behind DECNET_EMBED_SNIFFER (default off)

The API's lifespan unconditionally spawned a MACVLAN sniffer task, which
duplicated the standalone 'decnet sniffer --daemon' process that
'decnet deploy' always starts — causing two workers to sniff the same
interface, double events, and wasted CPU.

Mirror the existing DECNET_EMBED_PROFILER pattern: sniffer is OFF by
default, opt in explicitly. Static regression tests guard against
accidental removal of the gate.

tests/test_embedded_workers.py

@@ -0,0 +1,45 @@
+"""
+Regression guards for workers that duplicate standalone daemons.
+
+`decnet deploy` starts standalone `decnet sniffer --daemon` and
+`decnet profiler --daemon` processes. The API's lifespan must not spawn
+its own copies unless the operator explicitly opts in via env flags.
+
+These tests are intentionally static: we don't spin up lifespan, because
+scapy's sniff thread doesn't cooperate with asyncio cancellation and
+hangs pytest teardown.
+"""
+import importlib
+import inspect
+
+
+def test_embed_sniffer_defaults_off(monkeypatch):
+    monkeypatch.delenv("DECNET_EMBED_SNIFFER", raising=False)
+    import decnet.env
+    importlib.reload(decnet.env)
+    assert decnet.env.DECNET_EMBED_SNIFFER is False
+
+
+def test_embed_sniffer_flag_is_truthy_on_opt_in(monkeypatch):
+    monkeypatch.setenv("DECNET_EMBED_SNIFFER", "true")
+    import decnet.env
+    importlib.reload(decnet.env)
+    assert decnet.env.DECNET_EMBED_SNIFFER is True
+
+
+def test_api_lifespan_gates_sniffer_on_embed_flag():
+    """The lifespan source must reference the gate flag before spawning the
+    sniffer task — catches accidental removal of the guard in future edits."""
+    import decnet.web.api
+    src = inspect.getsource(decnet.web.api.lifespan)
+    assert "DECNET_EMBED_SNIFFER" in src, "sniffer gate removed from lifespan"
+    assert "sniffer_worker" in src
+    # Gate must appear before the task creation.
+    assert src.index("DECNET_EMBED_SNIFFER") < src.index("sniffer_worker")
+
+
+def test_api_lifespan_gates_profiler_on_embed_flag():
+    import decnet.web.api
+    src = inspect.getsource(decnet.web.api.lifespan)
+    assert "DECNET_EMBED_PROFILER" in src
+    assert src.index("DECNET_EMBED_PROFILER") < src.index("attacker_profile_worker")