firmware: add XIAO C6 sensor node

tests/test_xiao_sensor_node_contract.py

@@ -0,0 +1,27 @@
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+NODE = ROOT / "firmware" / "xiao-esp32c6-sensor-node"
+def source(name): return (NODE / name).read_text()
+
+def test_wiring_matches_assembly():
+    config = source("sensor_node_config.h")
+    expected = {"RS485_DE_PIN":"D2", "RS485_TX_PIN":"D4", "RS485_RX_PIN":"D5",
+                "BNO086_SDA_PIN":"D7", "BNO086_SCL_PIN":"D8", "GPS_RX_PIN":"D9", "GPS_TX_PIN":"D10"}
+    for name, pin in expected.items(): assert re.search(rf"#define\s+{name}\s+{pin}\b", config)
+
+def test_transport_contract():
+    text = source("rs485_transport.cpp")
+    for token in ['doc["type"] = "sensor_status"', 'doc["schema_version"]', 'createNestedObject("imu")', 'createNestedObject("gps")', "serial.write('\\n')", "serial.flush()"]:
+        assert token in text
+
+def test_loop_is_non_blocking():
+    loop = source("xiao-esp32c6-sensor-node.ino").split("void loop()", 1)[1]
+    assert "updateGps" in loop and "updateImu" in loop and "TELEMETRY_INTERVAL_MS" in loop
+    assert not re.search(r"delay\((?:[2-9]|[1-9]\d+)\)", loop)
+
+def test_node_does_not_take_cargo_or_ble_authority():
+    combined = "\n".join(p.read_text() for p in NODE.glob("*.*"))
+    assert "NimBLE" not in combined and "JBD" not in combined and "server.on(" not in combined
+