fix(fpga): F-0.1 wire AD9484 OR overrange pin into diagnostics

The AD9484 OR (overrange) LVDS pair is routed on the 50T main board to
xc7a50t-ftg256 bank-14 pins M6/N6 but was previously left unconnected at
the top level. Plumb it through the full stack so saturation at the raw
ADC boundary shows up in the existing overflow aggregation:

- ad9484_interface_400m: add adc_or_p/n inputs, IBUFDS + IDDR capture of
  both phases in the BUFIO domain, re-register into the clk_400m BUFG
  domain, OR rise|fall into adc_overrange_400m output.
- radar_receiver_final: stickify adc_overrange_400m in clk_400m, CDC to
  clk_100m via a 2FF ASYNC_REG chain (same reasoning as F-1.2's
  cdc_cic_fir_overrun — single-bit, latched low→high, GPIO-class
  diagnostic), OR into the existing ddc_overflow_any aggregation.
- radar_system_top: expose adc_or_p/n top-level ports and pass through.
- xc7a50t_ftg256.xdc: anchor M6/N6 as LVDS_25 DIFF_TERM, with the same
  DCO-relative input-delay constraints as adc_d_p[*].
- xc7a200t_fbg484.xdc: IOSTANDARD/DIFF_TERM set; PACKAGE_PIN left as a
  documented TODO — the 200T dev-board schematic has not been checked
  and the 200T build will need the anchor filled in before place/route.

9_Firmware/9_2_FPGA/constraints/xc7a200t_fbg484.xdc

@@ -134,6 +134,22 @@ set_property IOSTANDARD LVDS_25 [get_ports {adc_d_p[*]}]
 set_property IOSTANDARD LVDS_25 [get_ports {adc_d_n[*]}]
 set_property DIFF_TERM TRUE [get_ports {adc_d_p[*]}]
 
+# --------------------------------------------------------------------------
+# Audit F-0.1: AD9484 OR (overrange) LVDS pair
+# The 50T main board schematic routes ADC_OR_P/N to bank-14 pins M6/N6 on
+# xc7a50t-ftg256. The 200T dev-board schematic has NOT been checked yet;
+# adc_or_p/n are declared as top-level ports so the 50T build anchors them
+# cleanly, but the 200T anchor below is a TODO placeholder — synth/impl will
+# error on unplaced IO until the 200T schematic is verified and the PACKAGE_PIN
+# values are set. IOSTANDARD/DIFF_TERM properties stay as-is (same class as
+# adc_d_p).
+# --------------------------------------------------------------------------
+set_property IOSTANDARD LVDS_25 [get_ports {adc_or_p}]
+set_property IOSTANDARD LVDS_25 [get_ports {adc_or_n}]
+set_property DIFF_TERM TRUE [get_ports {adc_or_p}]
+# TODO(F-0.1): set_property PACKAGE_PIN <?> [get_ports {adc_or_p}] after 200T schematic audit
+# TODO(F-0.1): set_property PACKAGE_PIN <?> [get_ports {adc_or_n}] after 200T schematic audit
+
 # ADC Power Down — single-ended, Bank 14 (LVCMOS25 matches bank VCCO)
 # Pin: P20 = IO_0_14
 set_property PACKAGE_PIN P20 [get_ports {adc_pwdn}]