Add LNB polarity toggle (V-pol 13V / H-pol 18V)

Bridge: set_lnb_voltage(mode) wraps firmware lnbdc command, enable_lna()
now delegates to it. MCP: new set_lnb_voltage tool + 3 tests. TUI: Signal
screen button toggles between V-pol and H-pol instead of one-way LNA enable.

mcp/tests/test_signal.py

@@ -3,6 +3,7 @@
 import pytest
 from conftest import parse_result
 from fastmcp import Client
+from fastmcp.exceptions import ToolError
 
 
 @pytest.mark.anyio
@@ -42,6 +43,37 @@ async def test_enable_lna(mcp_client: Client):
     data = parse_result(result)
     assert data["status"] == "lna_enabled"
     assert data["voltage"] == "13V"
+    assert data["polarity"] == "V"
+
+
+@pytest.mark.anyio
+async def test_set_lnb_voltage_odu(mcp_client: Client):
+    result = await mcp_client.call_tool(
+        "set_lnb_voltage", {"mode": "odu"}
+    )
+    data = parse_result(result)
+    assert data["voltage"] == "13V"
+    assert data["polarity"] == "V"
+    assert data["mode"] == "odu"
+
+
+@pytest.mark.anyio
+async def test_set_lnb_voltage_stb(mcp_client: Client):
+    result = await mcp_client.call_tool(
+        "set_lnb_voltage", {"mode": "stb"}
+    )
+    data = parse_result(result)
+    assert data["voltage"] == "18V"
+    assert data["polarity"] == "H"
+    assert data["mode"] == "stb"
+
+
+@pytest.mark.anyio
+async def test_set_lnb_voltage_invalid(mcp_client: Client):
+    with pytest.raises(ToolError):
+        await mcp_client.call_tool(
+            "set_lnb_voltage", {"mode": "invalid"}
+        )
 
 
 @pytest.mark.anyio