fix: propagate FPGA AGC enable to MCU outer loop via DIG_6 GPIO

Resolve cross-layer AGC control mismatch where opcode 0x28 only
controlled the FPGA inner-loop AGC but the STM32 outer-loop AGC
(ADAR1000_AGC) ran independently with its own enable state.

FPGA: Drive gpio_dig6 from host_agc_enable instead of tied low,
making the FPGA register the single source of truth for AGC state.

MCU: Change ADAR1000_AGC constructor default from enabled(true) to
enabled(false) so boot state matches FPGA reset default (AGC off).
Read DIG_6 GPIO every frame with 2-frame confirmation debounce to
sync outerAgc.enabled — prevents single-sample glitch from causing
spurious AGC state transitions.

Tests: Update MCU unit tests for new default, add 6 cross-layer
contract tests verifying the FPGA-MCU-GUI AGC invariant chain.

9_Firmware/9_2_FPGA/radar_system_top.v

@@ -130,7 +130,7 @@ module radar_system_top (
     // FPGA→STM32 GPIO outputs (DIG_5..DIG_7 on 50T board)
     // Used by STM32 outer AGC loop to read saturation state without USB polling.
     output wire gpio_dig5,          // DIG_5 (H11→PD13): AGC saturation flag (1=clipping detected)
-    output wire gpio_dig6,          // DIG_6 (G12→PD14): reserved (tied low)
+    output wire gpio_dig6,          // DIG_6 (G12→PD14): AGC enable flag (mirrors host_agc_enable)
     output wire gpio_dig7           // DIG_7 (H12→PD15): reserved (tied low)
 );
 
@@ -1037,9 +1037,11 @@ assign system_status = status_reg;
 // ============================================================================
 // DIG_5: AGC saturation flag — high when per-frame saturation_count > 0.
 //        STM32 reads PD13 to detect clipping and adjust ADAR1000 VGA gain.
-// DIG_6, DIG_7: Reserved (tied low for future use).
+// DIG_6: AGC enable flag — mirrors host_agc_enable so STM32 outer-loop AGC
+//        tracks the FPGA register as single source of truth.
+// DIG_7: Reserved (tied low for future use).
 assign gpio_dig5 = (rx_agc_saturation_count != 8'd0);
-assign gpio_dig6 = 1'b0;
+assign gpio_dig6 = host_agc_enable;
 assign gpio_dig7 = 1'b0;
 
 // ============================================================================