feat(fpga): integrate 2048-pt FFT upgrade — non-conflicting RTL (wave 1/3)

File-scoped cherry-pick from feat/fft-2048-upgrade (e9705e4) for modules
that only the fft branch modified:

  RTL:
    cfar_ca.v                        512-row CFAR
    chirp_memory_loader_param.v      2-segment × 2048-sample loader
    doppler_processor.v              16384-deep doppler memory
    fft_engine.v                     2048-pt FFT
    matched_filter_multi_segment.v   2-seg overlap-save, BRAM overlap_cache
    matched_filter_processing_chain.v
    radar_mode_controller.v          XOR edge detector
    radar_params.vh                  (new) single source of truth
    range_bin_decimator.v            2048 -> 512 output bins
    rx_gain_control.v

  Memory:
    fft_twiddle_2048.mem             (new) 2048-pt FFT twiddles
    long_chirp_seg0_{i,q}.mem        2048-sample seg 0 (was 1024)
    long_chirp_seg1_{i,q}.mem        2048-sample seg 1 (was 1024)
    long_chirp_seg{2,3}_{i,q}.mem    deleted (4-seg -> 2-seg collapse)

  Gen:
    tb/cosim/gen_chirp_mem.py        regen script for mem files above

Waves 2 and 3 follow: manual merge for dual-modified files
(radar_system_top, usb_data_interface_ft2232h, mti_canceller,
radar_receiver_final), and CFAR pipeline from 2401f5f keeping p0's
CIC/DDC reset strategy.

9_Firmware/9_2_FPGA/fft_engine.v

@@ -28,13 +28,16 @@
  * Clock domain: single clock (clk), active-low async reset (reset_n).
  */
 
+// Include single source of truth for default parameters
+`include "radar_params.vh"
+
 module fft_engine #(
-    parameter N            = 1024,
-    parameter LOG2N        = 10,
+    parameter N            = `RP_FFT_SIZE,       // 2048
+    parameter LOG2N        = `RP_LOG2_FFT_SIZE,  // 11
     parameter DATA_W       = 16,
     parameter INTERNAL_W   = 32,
     parameter TWIDDLE_W    = 16,
-    parameter TWIDDLE_FILE = "fft_twiddle_1024.mem"
+    parameter TWIDDLE_FILE = "fft_twiddle_2048.mem"
 )(
     input wire clk,
     input wire reset_n,