E2E integration test + RTL fixes: mixer sequencing, USB data-pending flags, receiver toggle wiring (19/19 FPGA)

RTL fixes discovered via new end-to-end testbench:
- plfm_chirp_controller: TX/RX mixer enables now mutually exclusive
  by FSM state (Fix #4), preventing simultaneous TX+RX activation
- usb_data_interface: stream control reset default 3'b001 (range-only),
  added doppler/cfar data_pending sticky flags, write FSM triggers on
  range_valid only — eliminates startup deadlock (Fix #5)
- radar_receiver_final: STM32 toggle signals wired through for mode-00
  pass-through, dynamic frame detection via host_chirps_per_elev
- radar_system_top: STM32 toggle signal wiring to receiver instance
- chirp_memory_loader_param: explicit readmemh range for short chirp

Test infrastructure:
- New tb_system_e2e.v: 46 checks across 12 groups (reset, TX, safety,
  RX, USB R/W, CDC, beam scanning, reset recovery, stream control,
  latency budgets, watchdog)
- tb_usb_data_interface: Tests 21/22/56 updated for data_pending
  architecture (preload flags, verify consumption instead of state)
- tb_chirp_controller: mixer tests T7.1/T7.2 updated for Fix #4
- run_regression.sh: PASS/FAIL regex fixed to match only [PASS]/[FAIL]
  markers, added E2E test entry
- Updated rx_final_doppler_out.csv golden data

9_Firmware/9_2_FPGA/plfm_chirp_controller.v

@@ -80,9 +80,13 @@ assign elevation__toggling = new_elevation;
 assign azimuth__toggling = new_azimuth;
 assign new_chirp_frame = (current_state == IDLE && next_state == LONG_CHIRP);
 
-// Mixers Enabling
-assign rx_mixer_en = mixers_enable;
-assign tx_mixer_en = mixers_enable;
+// Mixer TX/RX sequencing — mutually exclusive based on chirp FSM state.
+// TX mixer active during chirp transmission, RX mixer during listen.
+// Both require mixers_enable (STM32 master enable) to be high.
+assign tx_mixer_en = mixers_enable && (current_state == LONG_CHIRP ||
+                                       current_state == SHORT_CHIRP);
+assign rx_mixer_en = mixers_enable && (current_state == LONG_LISTEN ||
+                                       current_state == SHORT_LISTEN);
 
 // ADTR1000 pull to ground tx and rx load pins if not used
 assign adar_tx_load_1 = 1'b0;