Add RPi fan controller, laptop temp sender, and Quickshell widget

Initial implementation of a USB-gadget-linked cooling controller:
- fan_controller.c: PWM PID control, tach reading, TCP temp receiver,
  Unix socket IPC, safe-mode fallback, CSV logging via pigpio.
- temp_sender.py: laptop-side lm-sensors/thermal_zone temp sender with
  auto-reconnect.
- FanController.qml: Quickshell floating widget for monitoring and
  setpoint/manual duty control over the Unix socket.

temp_sender.py

@@ -0,0 +1,29 @@
+import socket, time
+import subprocess
+
+RPI_IP   = "10.55.0.1"
+RPI_PORT = 9000
+INTERVAL = 2.0
+
+def get_temp():
+    out = subprocess.check_output(["sensors", "-u"]).decode()
+    # AMD Ryzen — Tctl is the control temp the firmware uses
+    for line in out.splitlines():
+        if "tctl" in line.lower() and "input" in line.lower():
+            return float(line.split(":")[1].strip())
+    # fallback to thermal_zone0
+    with open("/sys/class/thermal/thermal_zone0/temp") as f:
+        return int(f.read().strip()) / 1000.0
+
+# Auto-reconnect loop
+while True:
+    try:
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+            s.connect((RPI_IP, RPI_PORT))
+            print(f"Connected to {RPI_IP}:{RPI_PORT}")
+            while True:
+                s.sendall(f"{get_temp():.1f}\n".encode())
+                time.sleep(INTERVAL)
+    except (OSError, ConnectionResetError) as e:
+        print(f"Connection lost: {e} — retrying in 5s")
+        time.sleep(5)