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/src/birdcage_mcp/tools/signal.py

@@ -55,14 +55,42 @@ def register(mcp):
     async def enable_lna(ctx: Context) -> dict:
         """Enable the LNA by setting LNB voltage to 13V (V-pol).
 
-        This powers the low-noise amplifier in the dish's outdoor unit.
-        Boot default is 18V (H-pol). Required before meaningful RSSI
-        readings when used for radio astronomy.
+        Shortcut for set_lnb_voltage(mode='odu'). Use set_lnb_voltage
+        to switch back to 18V (H-pol) when done.
         """
         state: BirdcageState = ctx.request_context.lifespan_context
         _require_device(state)
         state.device.enable_lna()
-        return {"status": "lna_enabled", "voltage": "13V"}
+        return {"status": "lna_enabled", "voltage": "13V", "polarity": "V"}
+
+    @mcp.tool()
+    async def set_lnb_voltage(ctx: Context, mode: str) -> dict:
+        """Set LNB DC voltage and polarization.
+
+        Controls the LNB bias voltage on the coax feed line. This
+        determines both LNA power state and receive polarization:
+
+        - 'odu' = 13V (V-pol, LNA enabled) — for radio astronomy / ham
+        - 'stb' = 18V (H-pol, boot default) — standard consumer mode
+
+        V-pol and H-pol see different transponders. The PEAK submenu's
+        'rssits' command alternates between them for comparison.
+
+        Args:
+            mode: 'odu' for 13V/V-pol or 'stb' for 18V/H-pol.
+        """
+        state: BirdcageState = ctx.request_context.lifespan_context
+        _require_device(state)
+        raw = state.device.set_lnb_voltage(mode)
+        voltage = "13V" if mode.lower() == "odu" else "18V"
+        polarity = "V" if mode.lower() == "odu" else "H"
+        return {
+            "status": "set",
+            "mode": mode.lower(),
+            "voltage": voltage,
+            "polarity": polarity,
+            "raw": raw,
+        }
 
     @mcp.tool()
     async def get_dvb_config(ctx: Context) -> dict:

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