added remote k209 setup

docs/k209_setup.md

@@ -7,6 +7,11 @@ The production path is:
 K209 --USB-C--> S2VNA --HiSLIP/VISA--> radar_system
 ```
 
+For complete run-mode instructions, including what runs on the x86_64 S2VNA
+computer and what runs on Raspberry Pi, see
+[`docs/operation_modes.md`](operation_modes.md). For `run_config.json` fields,
+see [`docs/run_config.md`](run_config.md).
+
 There is no direct USB driver for K209 in this project. Do not use mock
 transports, socket fallbacks, or `pyvisa-py` for the K209 path. The required
 transport dependency is an IVI/Vendor VISA implementation that provides both
@@ -140,6 +145,31 @@ TCPIP0::127.0.0.1::hislip0,4880::INSTR
 If S2VNA runs on another machine, replace `127.0.0.1` with that machine's IP
 address.
 
+## Remote Raspberry Pi Mode
+
+For Raspberry Pi runs, keep S2VNA and NI-VISA on the x86_64 computer connected
+to the K209, and run only the project pipeline/GPIO on the Raspberry Pi.
+
+On the x86_64 computer with S2VNA running:
+
+```bash
+cd /path/to/radar_system
+.venv/bin/python -m python_app.scripts.k209_remote_server --host 0.0.0.0 --port 50209
+```
+
+On the Raspberry Pi, set the K209 config to the server address:
+
+```json
+"radar": {
+  "model": "compact_m_k209",
+  "remote_host": "192.168.1.10",
+  "remote_port": 50209,
+  "driver_mode": "native"
+}
+```
+
+The Raspberry Pi does not need S2VNA or NI-VISA for this mode.
+
 ## Python Smoke Test
 
 Use the project virtual environment:
@@ -263,9 +293,9 @@ are available for ARM64:
    TCPIP HiSLIP support.
 
 If those ARM64 dependencies are not available, run S2VNA and the acquisition
-process on an Ubuntu x86_64 machine. Raspberry Pi integration should then be
-handled at the system/pipeline level, not by replacing the K209 driver transport
-with a fallback.
+server on an Ubuntu x86_64 machine and use the remote K209 mode documented
+above. In that mode, Raspberry Pi runs the project pipeline and GPIO switch
+drivers, while the x86_64 machine runs S2VNA and the K209 remote server.
 
 ## Expected Hardware Test Result