fix(fpga): pre-bringup RTL hardening + test-suite hardening

RTL (P0 pre-bringup findings R-1/R-2/R-3/R-5/R-6):

- mti_canceller: add use_long_chirp input and waveform-boundary mute
  so the long->short transition in mode 01 no longer subtracts across
  heterogeneous waveforms (R-1). Prev buffer is overwritten in-flight
  at the boundary so the next same-waveform chirp subtracts cleanly.
- ad9484_interface_400m: 2FF sync of mmcm_locked into the 400 MHz
  domain before gating reset_n_gated (R-6).
- cic_decimator_4x_enhanced: correct max_fanout narrative (R-3).
- ad9484_interface_400m: strip stale pblock comment, note 3.0 ns
  max_delay instead (R-2).
- mti_canceller / doppler_processor: 200T-20km WARNING banners
  flagging the broken 4096-bin path (R-5). 9-bit BRAM address aliases
  silently until rewritten.
- adc_clk_mmcm.xdc: relax set_max_delay from 2.700 -> 3.000 ns,
  closes WNS with headroom on 50T build.
- radar_receiver_final: wire use_long_chirp into mti_inst.

Architecture-bump finalization (2048-pt range FFT, 512 range bins,
32 Doppler bins -> 16384 output cells per frame):

- tb/cosim/radar_scene.py: FFT_SIZE 1024 -> 2048, RANGE_BINS 64 -> 512.
- tb/gen_mf_golden_ref.py: N 1024 -> 2048.
- Regenerate all affected hex goldens (MF cases 1-4, Doppler inputs
  + py goldens, receiver integration golden_doppler.mem 2048 -> 16384).
- tb_radar_receiver_final: widen range_bin_out 6 -> 9 bits, bump
  GOLDEN_ENTRIES 2048 -> 16384, expand bitmaps/arrays to 512 bins,
  update all check messages and thresholds.
- tb_mti_canceller, tb_fullchain_mti_cfar_realdata: tie/pass
  use_long_chirp so compile still works after RTL port add.

Test-suite hardening (coverage audit findings):

- tb_mti_canceller T12: 10 new assertions exercising R-1 waveform-
  boundary mute across a long/long/short/short/long sequence. Catches
  a regression that re-enables subtraction across the boundary.
- tb_fir_lowpass: replace tautological check(1'b1, ...) on coefficient
  symmetry with a real hierarchical check coeff[k]===coeff[31-k];
  replace always-pass overflow check with a well-driven (not X/Z)
  assertion on filter_overflow.
- tb_matched_filter_processing_chain: replace three always-pass peak-
  bin placeholders with peak-to-mean-|out| > 2x ratio checks (catches
  flat/zero output that the old tautologies silently accepted).
- tb_cdc_modules M2: replace always-pass narrow-pulse check with a
  well-defined-output assertion on the synchronizer.
- tb_nco_400m: replace always-pass freq-switch check with a swing +
  no-X assertion across 200 post-switch samples.
- tb_system_e2e G12.1: replace check(1, ...) with test_num > 20 so
  it catches a stalled TB that skipped prior groups.
- tb_multiseg_cosim TEST 4: replace always-pass placeholder with a
  bitmap that asserts segment_request visited all 4 values.
- tb_mf_chain_synth and tb_fullchain_mti_cfar_realdata: add DEPRECATED
  headers plus \$fatal guards (ifndef ALLOW_STALE_*) so they cannot
  be silently re-enabled in CI with stale 1024-bin goldens against
  current 2048-pt RTL.

Regression: 32 passed, 0 failed. MTI TB grew 30 -> 39 checks;
receiver integration grew 17 -> 18 checks with 16384/16384 golden
match at tolerance +/- 2 LSB.

9_Firmware/9_2_FPGA/tb/cosim/radar_scene.py

@@ -52,9 +52,11 @@ T_PRI_SHORT = 175e-6      # staggered short-PRI sub-frame
 N_SAMPLES_LISTEN = int(T_LISTEN_LONG * FS_ADC)  # 54800 samples
 
 # Processing chain
+# Updated for 2048-pt range FFT + 4x decimation → 512 range bins per chirp.
+# Must stay in sync with radar_params.vh: RP_FFT_SIZE=2048, RP_NUM_RANGE_BINS=512.
 CIC_DECIMATION = 4
-FFT_SIZE = 1024
-RANGE_BINS = 64
+FFT_SIZE = 2048
+RANGE_BINS = 512
 DOPPLER_FFT_SIZE = 16      # Per sub-frame
 DOPPLER_TOTAL_BINS = 32    # Total output bins (2 sub-frames x 16)
 CHIRPS_PER_SUBFRAME = 16