build: vendor the Slamtec rplidar SDK in third_party

The repo is now self-contained: driver_rplidar compiles the SDK from
third_party/rplidar_sdk (Linux sources only, teardown delete[] bug fixed
in place), so a plain cmake configure needs no SDK path and no probing —
the XLIDAR_RPLIDAR_SDK_DIR option and external references are gone.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugins/driver_rplidar/rplidar_driver.cpp

@@ -1,11 +1,10 @@
-// Slamtec RPLIDAR over serial (C1 defaults), built on the vendor SDK
-// (sl_lidar.h). Ported from xlocd's embedded rplidar driver — the scan math
-// (angle/distance decoding, inversion, FOV window, NaN invalid points) is
-// kept identical.
+// Slamtec RPLIDAR over serial (C1 defaults), built on the vendored SDK
+// (third_party/rplidar_sdk, sl_lidar.h). Scan math: angle/distance decoding,
+// inversion, FOV window, invalid points as NaN.
 //
 // Unlike the network drivers, angles are reported in the DEVICE frame
 // [0, 2π), 0 = ahead, ascending — exactly what the SDK's ascendScanData
-// yields (and what xlocd's engine expects).
+// yields.
 #include "lidar_interface.hpp"
 #include "plugin_helpers.hpp"