merge: resolve conflicts with develop (supersede by PR #89 / #107)

Three conflicts — all resolved in favor of develop, which has a more
refined version of the same work this branch introduced:

- radar_system_top.v: develop's cleaner USB_MODE=1 comment (same value).
- run_regression.sh: develop's ${SYSTEM_RTL[@]} refactor + added
  USB_MODE=1 test variants.
- tb/radar_system_tb.v: develop's ifdef USB_MODE_1 to dump the correct
  USB instance based on mode.

The 400 MHz reset fan-out fix (nco_400m_enhanced, cic_decimator_4x_enhanced,
ddc_400m) and ADAR1000 channel-indexing fix remain intact on this branch.

9_Firmware/9_1_Microcontroller/9_1_1_C_Cpp_Libraries/diag_log.h

@@ -112,7 +112,7 @@ extern "C" {
  *   "BF"    -- ADAR1000 beamformer
  *   "PA"    -- Power amplifier bias/monitoring
  *   "FPGA"  -- FPGA communication and handshake
- *   "USB"   -- FT601 USB data path
+ *   "USB"   -- USB data path (FT2232H production / FT601 premium)
  *   "PWR"   -- Power sequencing and rail monitoring
  *   "IMU"   -- IMU/GPS/barometer sensors
  *   "MOT"   -- Stepper motor/scan mechanics