feat: 2048-pt FFT upgrade with decimation=4, 512 output bins, 6m spacing

Complete cross-layer upgrade from 1024-pt/64-bin to 2048-pt/512-bin FFT:

FPGA RTL (14+ modules):
- radar_params.vh: FFT_SIZE=2048, RANGE_BINS=512, 9-bit range, 6-bit stream
- fft_engine.v: 2048-pt FFT with XPM BRAM
- chirp_memory_loader_param.v: 2 segments x 2048 (was 4 x 1024)
- matched_filter_multi_segment.v: BRAM inference for overlap_cache, explicit ov_waddr
- mti_canceller.v: BRAM inference for prev_i/q arrays (was fabric FFs)
- doppler_processor.v: 16384-deep memory, 14-bit addressing
- cfar_ca.v: 512 rows, indentation fix
- radar_receiver_final.v: rising-edge detector for frame_complete, 11-bit sample_addr
- range_bin_decimator.v: 512 output bins
- usb_data_interface_ft2232h.v: bulk per-frame with Manhattan magnitude
- radar_mode_controller.v: XOR edge detector for toggle signals
- rx_gain_control.v: updated for new bin count

Python GUI + Protocol (8 files):
- radar_protocol.py: 512-bin bulk frame parser, LSB-first bitmap
- GUI_V65_Tk.py, v7/*.py: updated for 512 bins, 6m range resolution

Golden data + tests:
- All .hex/.csv/.npy golden references regenerated for 2048/512
- fft_twiddle_2048.mem added
- Deleted stale seg2/seg3 chirp mem files
- 9 new bulk frame cross-layer tests, deleted 6 stale per-sample tests
- Deleted stale tb_cross_layer_ft2232h.v and dead contract_parser functions
- Updated validate_mem_files.py for 2048/2-segment config

MCU: RadarSettings.cpp max_distance/map_size 1536->3072

All 4 CI jobs pass: 285 tests, 0 failures, 0 skips

9_Firmware/9_3_GUI/v7/models.py

@@ -109,11 +109,11 @@ class RadarSettings:
     the actual waveform parameters.
     """
     system_frequency: float = 10.5e9    # Hz (PLFM TX LO, verified from ADF4382 config)
-    range_bin_spacing: float = 24.0    # Meters per decimated range bin (c/(2*100MSPS)*16)
+    range_bin_spacing: float = 6.0      # Meters per decimated range bin (c/(2*100MSPS)*4)
     velocity_resolution: float = 2.67  # m/s per Doppler bin (lam/(2*32*167us))
-    max_distance: float = 1536         # Max detection range (m) -- 64 bins x 24 m (3 km mode)
-    map_size: float = 1536             # Map display size (m)
-    coverage_radius: float = 1536      # Map coverage radius (m)
+    max_distance: float = 3072         # Max detection range (m) -- 512 bins x 6 m (3 km mode)
+    map_size: float = 3072             # Map display size (m)
+    coverage_radius: float = 3072      # Map coverage radius (m)
 
 
 @dataclass
@@ -210,10 +210,10 @@ class WaveformConfig:
     chirp_duration_s: float = 30e-6      # Long chirp ramp (informational only)
     pri_s: float = 167e-6               # Pulse repetition interval (chirp + listen)
     center_freq_hz: float = 10.5e9       # TX LO carrier (verified: ADF4382 config)
-    n_range_bins: int = 64               # After decimation (3 km mode)
+    n_range_bins: int = 512              # After decimation (3 km mode)
     n_doppler_bins: int = 32             # After Doppler FFT
-    fft_size: int = 1024                 # Pre-decimation FFT length
-    decimation_factor: int = 16          # 1024 → 64
+    fft_size: int = 2048                 # Pre-decimation FFT length
+    decimation_factor: int = 4           # 2048 → 512
 
     @property
     def bin_spacing_m(self) -> float: