telemetry: diff-based CSV logger with ZMQ IPC

- telemetry.py: client library, tlog(group, data) sends JSON over ZMQ PUSH
- telemetryd.py: collector process, diffs against previous state per group,
  writes only changed values to /data/log2/{session}/telemetry.csv
- Registered as always-run managed process in process_config.py

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

selfdrive/manager/process_config.py

@@ -109,6 +109,7 @@ procs = [
 
   # ClearPilot processes
   NativeProcess("dashcamd", "selfdrive/clearpilot", ["./dashcamd"], dashcam_should_run),
+  PythonProcess("telemetryd", "selfdrive.clearpilot.telemetryd", always_run),
 ]
 
 managed_processes = {p.name: p for p in procs}