fix: use authoritative tx frame signal for frame sync, consistent ad9523 error path

FPGA-001: The previous fix derived frame boundaries from chirp_counter==0,
but that counter comes from plfm_chirp_controller_enhanced which overflows
to N (not wrapping at chirps_per_elev). This caused frame pulses only on
6-bit rollover (every 64 chirps) instead of every N chirps. Now wires the
CDC-synchronized tx_new_chirp_frame_sync signal from the transmitter into
radar_receiver_final, giving correct per-frame timing for any N.

STM32-004: Changed ad9523_init() failure path from Error_Handler() to
return -1, matching the pattern used by ad9523_setup() and ad9523_status()
in the same function. Both halt the system, but return -1 keeps IRQs
enabled for diagnostic output.

9_Firmware/9_1_Microcontroller/9_1_3_C_Cpp_Code/main.cpp

@@ -1181,8 +1181,8 @@ static int configure_ad9523(void)
         int32_t init_ret = ad9523_init(&init_param);
         DIAG("CLK", "ad9523_init() returned %ld", (long)init_ret);
         if (init_ret != 0) {
-            DIAG_ERR("CLK", "ad9523_init() FAILED (ret=%ld) -- calling Error_Handler()", (long)init_ret);
-            Error_Handler();
+            DIAG_ERR("CLK", "ad9523_init() FAILED (ret=%ld)", (long)init_ret);
+            return -1;
         }
     }