feat(relay): register RelayAdapter through platform registry (flagged off by default)

register_relay_adapter() registers the generic 'relay' platform via the same
PlatformRegistry path as plugin adapters — no core dispatch changes. OFF by
default (dark-launch): only registers when HERMES_GATEWAY_RELAY is truthy (or
force=True for tests), so existing single-tenant/direct deployments are
unaffected. Factory builds a transport-less RelayAdapter with a placeholder
descriptor; the real descriptor is negotiated at handshake.

Phase 1, Task 1.3 of the gateway-relay plan.

tests/gateway/relay/test_relay_registration.py

@@ -0,0 +1,55 @@
+"""RelayAdapter registration via the platform registry (relay Phase 1, Task 1.3).
+
+Verifies the relay platform is registered ONLY behind the flag (dark-launch),
+constructed through the same registry path as plugin adapters.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from gateway.config import PlatformConfig
+from gateway.platform_registry import platform_registry
+from gateway.relay import register_relay_adapter, relay_enabled
+from gateway.relay.adapter import RelayAdapter
+
+
+@pytest.fixture(autouse=True)
+def _clean_registry(monkeypatch):
+    """Ensure each test starts/ends with no 'relay' entry and a clean env."""
+    monkeypatch.delenv("HERMES_GATEWAY_RELAY", raising=False)
+    platform_registry.unregister("relay")
+    yield
+    platform_registry.unregister("relay")
+
+
+def test_off_by_default():
+    assert relay_enabled() is False
+    assert register_relay_adapter() is False
+    assert platform_registry.is_registered("relay") is False
+
+
+def test_enabled_by_env_flag(monkeypatch):
+    monkeypatch.setenv("HERMES_GATEWAY_RELAY", "1")
+    assert relay_enabled() is True
+    assert register_relay_adapter() is True
+    assert platform_registry.is_registered("relay") is True
+
+
+def test_force_registers_without_flag():
+    assert register_relay_adapter(force=True) is True
+    assert platform_registry.is_registered("relay") is True
+
+
+def test_create_adapter_yields_relay_adapter():
+    register_relay_adapter(force=True)
+    adapter = platform_registry.create_adapter("relay", PlatformConfig())
+    assert isinstance(adapter, RelayAdapter)
+    # Placeholder descriptor until handshake negotiates the real one.
+    assert adapter.descriptor.platform == "relay"
+
+
+@pytest.mark.parametrize("val,expected", [("0", False), ("", False), ("true", True), ("ON", True), ("yes", True)])
+def test_flag_parsing(monkeypatch, val, expected):
+    monkeypatch.setenv("HERMES_GATEWAY_RELAY", val)
+    assert relay_enabled() is expected