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/radar_system_top.v

@@ -142,7 +142,7 @@ module radar_system_top (
 parameter USE_LONG_CHIRP = 1'b1;          // Default to long chirp
 parameter DOPPLER_ENABLE = 1'b1;           // Enable Doppler processing
 parameter USB_ENABLE = 1'b1;               // Enable USB data transfer
-parameter USB_MODE = 0;                    // 0=FT601 (32-bit, 200T), 1=FT2232H (8-bit, 50T)
+parameter USB_MODE = 1;                    // 0=FT601 (32-bit, 200T), 1=FT2232H (8-bit, 50T) — default: FT2232H production board
 
 // ============================================================================
 // INTERNAL SIGNALS
@@ -243,12 +243,12 @@ reg [5:0]  host_chirps_per_elev;      // Opcode 0x15 (default 32)
 reg        host_status_request;       // Opcode 0xFF (self-clearing pulse)
 
 // Fix 4: Doppler/chirps mismatch protection
-// DOPPLER_FRAME_CHIRPS is the fixed chirp count expected by the staggered-PRI
-// Doppler path (16 long + 16 short). If host sets chirps_per_elev to a
-// different value, Doppler accumulation is corrupted. Clamp at command decode
-// and flag the mismatch so the host knows.
-localparam DOPPLER_FRAME_CHIRPS = 32; // Total chirps per Doppler frame
-reg        chirps_mismatch_error;     // Set if host tried to set chirps != FFT size
+// DOPPLER_FRAME_CHIRPS is the fixed chirp count expected by the staggered-PRI
+// Doppler path (16 long + 16 short). If host sets chirps_per_elev to a
+// different value, Doppler accumulation is corrupted. Clamp at command decode
+// and flag the mismatch so the host knows.
+localparam DOPPLER_FRAME_CHIRPS = 32; // Total chirps per Doppler frame
+reg        chirps_mismatch_error;     // Set if host tried to set chirps != FFT size
 
 // Fix 7: Range-mode register (opcode 0x20)
 // Future-proofing for 3km/10km antenna switching.
@@ -578,21 +578,21 @@ assign rx_doppler_data_valid = rx_doppler_valid;
 // ============================================================================
 // DC NOTCH FILTER (post-Doppler-FFT, pre-CFAR)
 // ============================================================================
-// Zeros out Doppler bins within ±host_dc_notch_width of DC for BOTH
-// sub-frames in the dual 16-pt FFT architecture.
-// doppler_bin[4:0] = {sub_frame, bin[3:0]}:
-//   Sub-frame 0: bins 0-15,  DC = bin 0,  wrap = bin 15
-//   Sub-frame 1: bins 16-31, DC = bin 16, wrap = bin 31
-// notch_width=1 → zero bins {0,16}. notch_width=2 → zero bins
-// {0,1,15,16,17,31}. etc.
-// When host_dc_notch_width=0: pass-through (no zeroing).
-
-wire dc_notch_active;
-wire [4:0] dop_bin_unsigned = rx_doppler_bin;
-wire [3:0] bin_within_sf = dop_bin_unsigned[3:0];
-assign dc_notch_active = (host_dc_notch_width != 3'd0) &&
-                          (bin_within_sf < {1'b0, host_dc_notch_width} ||
-                           bin_within_sf > (4'd15 - {1'b0, host_dc_notch_width} + 4'd1));
+// Zeros out Doppler bins within ±host_dc_notch_width of DC for BOTH
+// sub-frames in the dual 16-pt FFT architecture.
+// doppler_bin[4:0] = {sub_frame, bin[3:0]}:
+//   Sub-frame 0: bins 0-15,  DC = bin 0,  wrap = bin 15
+//   Sub-frame 1: bins 16-31, DC = bin 16, wrap = bin 31
+// notch_width=1 → zero bins {0,16}. notch_width=2 → zero bins
+// {0,1,15,16,17,31}. etc.
+// When host_dc_notch_width=0: pass-through (no zeroing).
+
+wire dc_notch_active;
+wire [4:0] dop_bin_unsigned = rx_doppler_bin;
+wire [3:0] bin_within_sf = dop_bin_unsigned[3:0];
+assign dc_notch_active = (host_dc_notch_width != 3'd0) &&
+                          (bin_within_sf < {1'b0, host_dc_notch_width} ||
+                           bin_within_sf > (4'd15 - {1'b0, host_dc_notch_width} + 4'd1));
 
 // Notched Doppler data: zero I/Q when in notch zone, pass through otherwise
 wire [31:0] notched_doppler_data  = dc_notch_active ? 32'd0 : rx_doppler_output;
@@ -959,19 +959,19 @@ always @(posedge clk_100m_buf or negedge sys_reset_n) begin
                 8'h13: host_short_chirp_cycles <= usb_cmd_value;
                 8'h14: host_short_listen_cycles <= usb_cmd_value;
                 8'h15: begin
-                    // Fix 4: Clamp chirps_per_elev to the fixed Doppler frame size.
-                    // If host requests a different value, clamp and set error flag.
-                    if (usb_cmd_value[5:0] > DOPPLER_FRAME_CHIRPS[5:0]) begin
-                        host_chirps_per_elev  <= DOPPLER_FRAME_CHIRPS[5:0];
-                        chirps_mismatch_error <= 1'b1;
-                    end else if (usb_cmd_value[5:0] == 6'd0) begin
-                        host_chirps_per_elev  <= DOPPLER_FRAME_CHIRPS[5:0];
-                        chirps_mismatch_error <= 1'b1;
-                    end else begin
-                        host_chirps_per_elev  <= usb_cmd_value[5:0];
-                        // Clear error only if value matches FFT size exactly
-                        chirps_mismatch_error <= (usb_cmd_value[5:0] != DOPPLER_FRAME_CHIRPS[5:0]);
-                    end
+                    // Fix 4: Clamp chirps_per_elev to the fixed Doppler frame size.
+                    // If host requests a different value, clamp and set error flag.
+                    if (usb_cmd_value[5:0] > DOPPLER_FRAME_CHIRPS[5:0]) begin
+                        host_chirps_per_elev  <= DOPPLER_FRAME_CHIRPS[5:0];
+                        chirps_mismatch_error <= 1'b1;
+                    end else if (usb_cmd_value[5:0] == 6'd0) begin
+                        host_chirps_per_elev  <= DOPPLER_FRAME_CHIRPS[5:0];
+                        chirps_mismatch_error <= 1'b1;
+                    end else begin
+                        host_chirps_per_elev  <= usb_cmd_value[5:0];
+                        // Clear error only if value matches FFT size exactly
+                        chirps_mismatch_error <= (usb_cmd_value[5:0] != DOPPLER_FRAME_CHIRPS[5:0]);
+                    end
                 end
                 8'h16: host_gain_shift         <= usb_cmd_value[3:0];  // Fix 3: digital gain
                 8'h20: host_range_mode         <= usb_cmd_value[1:0];  // Fix 7: range mode
@@ -1075,4 +1075,4 @@ always @(posedge clk_100m_buf) begin
 end
 `endif
 
-endmodule
+endmodule