feat(fpga): make FT2232H default USB interface, rewrite FT601 write FSM, add clock-loss watchdog

- Set USB_MODE default to 1 (FT2232H) in radar_system_top.v; 200T build
  overrides to USB_MODE=0 via build_200t.tcl generic property
- Rewrite FT601 write FSM: 4-state architecture with 3-word packed data,
  pending-flag gating, and frame sync counter
- Add FT2232H read FSM rd_cmd_complete flag, stream field zeroing, and
  range_data_ready 1-cycle pipeline delay in both USB modules
- Implement clock-loss watchdog: ft_heartbeat toggle + 16-bit timeout
  counter drives ft_clk_lost, feeding ft_effective_reset_n via 2-stage
  ASYNC_REG synchronizer chain
- Fix sample_counter reset literal width (11'd0 -> 12'd0)
- Add FT2232H I/O timing constraints to 50T XDC; fix dac_clk comments
- Document vestigial ft601_txe_n/rxf_n ports (needed for 200T XDC)
- Tie off AGC ports on TE0713 dev wrapper
- Rewrite tb_usb_data_interface.v for new 4-state FSM (89 checks)
- Add USB_MODE=1 regression runs; remove dead CHECK 5/6 loop
- Update diag_log.h USB interface comment

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