test: add palace_graph tunnel helper coverage

Adds focused tests for explicit tunnel helpers in `mempalace/palace_graph.py`.

Covered:
- `_load_tunnels`
- `_save_tunnels`
- `create_tunnel`
- `list_tunnels`
- `delete_tunnel`
- `follow_tunnels`

tests/test_palace_graph_tunnels.py

@@ -0,0 +1,137 @@
+"""Tests for explicit tunnel helpers in mempalace.palace_graph."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+with patch.dict("sys.modules", {"chromadb": MagicMock()}):
+    import mempalace.palace_graph as palace_graph
+
+
+def _use_tmp_tunnel_file(monkeypatch, tmp_path):
+    tunnel_file = tmp_path / "tunnels.json"
+    monkeypatch.setattr(palace_graph, "_TUNNEL_FILE", str(tunnel_file))
+    return tunnel_file
+
+
+class TestTunnelStorage:
+    def test_load_tunnels_missing_file_returns_empty_list(self, tmp_path, monkeypatch):
+        _use_tmp_tunnel_file(monkeypatch, tmp_path)
+        assert palace_graph._load_tunnels() == []
+
+    def test_load_tunnels_corrupt_file_returns_empty_list(self, tmp_path, monkeypatch):
+        tunnel_file = _use_tmp_tunnel_file(monkeypatch, tmp_path)
+        tunnel_file.write_text("{not valid json", encoding="utf-8")
+        assert palace_graph._load_tunnels() == []
+
+    def test_save_and_load_round_trip(self, tmp_path, monkeypatch):
+        _use_tmp_tunnel_file(monkeypatch, tmp_path)
+        tunnels = [
+            {
+                "id": "abc123",
+                "source": {"wing": "wing_code", "room": "auth"},
+                "target": {"wing": "wing_people", "room": "users"},
+                "label": "same concept",
+            }
+        ]
+        palace_graph._save_tunnels(tunnels)
+        assert palace_graph._load_tunnels() == tunnels
+
+
+class TestExplicitTunnels:
+    def test_create_tunnel_deduplicates_reverse_order_and_updates_label(
+        self, tmp_path, monkeypatch
+    ):
+        _use_tmp_tunnel_file(monkeypatch, tmp_path)
+
+        first = palace_graph.create_tunnel(
+            "wing_code", "auth", "wing_people", "users", label="same concept"
+        )
+        second = palace_graph.create_tunnel(
+            "wing_people", "users", "wing_code", "auth", label="updated label"
+        )
+
+        assert first["id"] == second["id"]
+        assert len(palace_graph.list_tunnels()) == 1
+        assert second["label"] == "updated label"
+        assert second["created_at"] == first["created_at"]
+        assert "updated_at" in second
+
+    def test_create_tunnel_rejects_empty_names(self, tmp_path, monkeypatch):
+        _use_tmp_tunnel_file(monkeypatch, tmp_path)
+
+        with pytest.raises(ValueError):
+            palace_graph.create_tunnel("", "auth", "wing_people", "users")
+
+    def test_list_tunnels_filters_by_either_side(self, tmp_path, monkeypatch):
+        _use_tmp_tunnel_file(monkeypatch, tmp_path)
+
+        palace_graph.create_tunnel("wing_code", "auth", "wing_people", "users", label="A")
+        palace_graph.create_tunnel("wing_ops", "deploy", "wing_people", "users", label="B")
+
+        assert len(palace_graph.list_tunnels()) == 2
+        assert len(palace_graph.list_tunnels("wing_people")) == 2
+        assert len(palace_graph.list_tunnels("wing_code")) == 1
+
+    def test_delete_tunnel_removes_saved_tunnel(self, tmp_path, monkeypatch):
+        _use_tmp_tunnel_file(monkeypatch, tmp_path)
+
+        tunnel = palace_graph.create_tunnel(
+            "wing_code", "auth", "wing_people", "users", label="same concept"
+        )
+
+        assert palace_graph.delete_tunnel(tunnel["id"]) == {"deleted": tunnel["id"]}
+        assert palace_graph.list_tunnels() == []
+
+    def test_follow_tunnels_returns_direction_and_preview(self, tmp_path, monkeypatch):
+        _use_tmp_tunnel_file(monkeypatch, tmp_path)
+
+        palace_graph.create_tunnel(
+            "wing_code",
+            "auth",
+            "wing_people",
+            "users",
+            label="same concept",
+            target_drawer_id="drawer_users_1",
+        )
+
+        col = MagicMock()
+        col.get.return_value = {
+            "ids": ["drawer_users_1"],
+            "documents": ["A" * 400],
+            "metadatas": [{}],
+        }
+
+        outgoing = palace_graph.follow_tunnels("wing_code", "auth", col=col)
+        assert len(outgoing) == 1
+        assert outgoing[0]["direction"] == "outgoing"
+        assert outgoing[0]["connected_wing"] == "wing_people"
+        assert outgoing[0]["connected_room"] == "users"
+        assert outgoing[0]["drawer_id"] == "drawer_users_1"
+        assert len(outgoing[0]["drawer_preview"]) == 300
+
+        incoming = palace_graph.follow_tunnels("wing_people", "users", col=col)
+        assert len(incoming) == 1
+        assert incoming[0]["direction"] == "incoming"
+        assert incoming[0]["connected_wing"] == "wing_code"
+
+    def test_follow_tunnels_returns_connections_even_if_collection_lookup_fails(
+        self, tmp_path, monkeypatch
+    ):
+        _use_tmp_tunnel_file(monkeypatch, tmp_path)
+
+        palace_graph.create_tunnel(
+            "wing_code",
+            "auth",
+            "wing_people",
+            "users",
+            label="same concept",
+            target_drawer_id="drawer_users_1",
+        )
+
+        col = MagicMock()
+        col.get.side_effect = RuntimeError("boom")
+
+        connections = palace_graph.follow_tunnels("wing_code", "auth", col=col)
+        assert len(connections) == 1
+        assert "drawer_preview" not in connections[0]