fix(fpga): registered reset fan-out at 400 MHz; default USB to FT2232H

Replace direct !reset_n async sense with a registered active-high reset_h
(max_fanout=50) in nco_400m_enhanced, cic_decimator_4x_enhanced, and
ddc_400m.  The prior single-LUT1 / 700+ load net was the root cause of
WNS=-0.626 ns in the 400 MHz clock domain on the xc7a50t build.  Vivado
replicates the constrained register into ≈14 regional copies, each driving
≤50 loads, closing timing at 2.5 ns.

Change radar_system_top default USB_MODE from 0 (FT601) to 1 (FT2232H).
FT601 remains available for the 200T premium board via explicit parameter
override; the 50T production wrapper already hard-codes USB_MODE=1.

Regression: add usb_data_interface_ft2232h.v to PROD_RTL lint list and
both system-top TB compile commands; fix legacy radar_system_tb hierarchical
probe from gen_ft601.usb_inst to gen_ft2232h.usb_inst.

Golden reference files (rtl_bb_dc.csv, rx_final_doppler_out.csv,
golden_doppler.mem) regenerated to reflect the +1-cycle registered-reset
boundary behaviour; Receiver golden-compare passes 18/18 checks.

All 25 regression tests pass (0 failures, 0 skipped).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

9_Firmware/9_2_FPGA/tb/radar_system_tb.v

@@ -619,7 +619,7 @@ initial begin
     // Optional: dump specific signals for debugging
     $dumpvars(1, dut.tx_inst);
     $dumpvars(1, dut.rx_inst);
-    $dumpvars(1, dut.gen_ft601.usb_inst);
+    $dumpvars(1, dut.gen_ft2232h.usb_inst);
 end
 
 endmodule