fix(rtl): add DONT_TOUCH attribute to prevent opt_design from gutting 50T wrapper

Build attempt 10 produced a valid bitstream but with only 315 LUTs and
15 DSPs — opt_design removed all logic feeding unconnected _nc wires.
Adding (* DONT_TOUCH = "TRUE" *) on the u_core instance prevents
Vivado from optimizing away the internal radar pipeline logic.

9_Firmware/9_2_FPGA/radar_system_top_50t.v

@@ -95,6 +95,7 @@ module radar_system_top_50t (
     wire [5:0]  dbg_range_bin_nc;
     wire [3:0]  system_status_nc;
 
+    (* DONT_TOUCH = "TRUE" *)
     radar_system_top u_core (
         // ----- Clocks & Reset -----
         .clk_100m               (clk_100m),