feat: hybrid AGC (FPGA phases 1-3 + GUI phase 6) with timing fix

FPGA:
- rx_gain_control.v rewritten: per-frame peak/saturation tracking,
  auto-shift AGC with attack/decay/holdoff, signed gain -7 to +7
- New registers 0x28-0x2C (agc_enable/target/attack/decay/holdoff)
- status_words[4] carries AGC metrics (gain, peak, sat_count, enable)
- DIG_5 GPIO outputs saturation flag for STM32 outer loop
- Both USB interfaces (FT601 + FT2232H) updated with AGC status ports

Timing fix (WNS +0.001ns -> +0.045ns, 45x improvement):
- CIC max_fanout 4->16 on valid pipeline registers
- +200ps setup uncertainty on 400MHz domain
- ExtraNetDelay_high placement + AggressiveExplore routing

GUI:
- AGC opcodes + status parsing in radar_protocol.py
- AGC control groups in both tkinter and V7 PyQt dashboards
- 11 new AGC tests (103/103 GUI tests pass)

Cross-layer:
- AGC opcodes/defaults/status assertions added (29/29 pass)
- contract_parser.py: fixed comment stripping in concat parser

All tests green: 25 FPGA + 103 GUI + 29 cross-layer = 157 pass

9_Firmware/9_2_FPGA/usb_data_interface.v

@@ -77,7 +77,13 @@ module usb_data_interface (
     // Self-test status readback (opcode 0x31 / included in 0xFF status packet)
     input wire [4:0]  status_self_test_flags,  // Per-test PASS(1)/FAIL(0) latched
     input wire [7:0]  status_self_test_detail, // Diagnostic detail byte latched
-    input wire        status_self_test_busy    // Self-test FSM still running
+    input wire        status_self_test_busy,   // Self-test FSM still running
+
+    // AGC status readback
+    input wire [3:0]  status_agc_current_gain,
+    input wire [7:0]  status_agc_peak_magnitude,
+    input wire [7:0]  status_agc_saturation_count,
+    input wire        status_agc_enable
 );
 
 // USB packet structure (same as before)
@@ -267,8 +273,13 @@ always @(posedge ft601_clk_in or negedge ft601_reset_n) begin
             status_words[2] <= {status_guard, status_short_chirp};
             // Word 3: {short_listen_cycles[15:0], chirps_per_elev[5:0], 10'b0}
             status_words[3] <= {status_short_listen, 10'd0, status_chirps_per_elev};
-            // Word 4: Fix 7 — range_mode in bits [1:0], rest reserved
-            status_words[4] <= {30'd0, status_range_mode};
+            // Word 4: AGC metrics + range_mode
+            status_words[4] <= {status_agc_current_gain,        // [31:28]
+                                status_agc_peak_magnitude,      // [27:20]
+                                status_agc_saturation_count,    // [19:12]
+                                status_agc_enable,              // [11]
+                                9'd0,                           // [10:2] reserved
+                                status_range_mode};             // [1:0]
             // Word 5: Self-test results {reserved[6:0], busy, reserved[7:0], detail[7:0], reserved[2:0], flags[4:0]}
             status_words[5] <= {7'd0, status_self_test_busy,
                                 8'd0, status_self_test_detail,