fix: 9 bugs from code review — RTL sign-ext & snapshot, thread safety, protocol fixes

- rx_gain_control.v: sign-extension fix ({agc_gain[3],agc_gain} not {1'b0,agc_gain})
  + inclusive frame_boundary snapshot via combinational helpers (Bug #7)
- v7/dashboard.py: Qt thread-safe logging via pyqtSignal bridge (Bug #1)
  + table headers corrected to 'Range (m)' / 'Velocity (m/s)' (Bug #2)
- main.cpp: guard outerAgc.applyGain() with if(outerAgc.enabled) (Bug #3)
- radar_protocol.py: replay L1 threshold detection when CFAR disabled (Bug #4)
  + IndexError guard in replay open (Bug #5) + AGC opcodes in _HARDWARE_ONLY_OPCODES
- radar_dashboard.py: AGC monitor attribute name fixes (3 labels)
- tb_rx_gain_control.v: Tests 17-19 (sign-ext, simultaneous valid+boundary, enable toggle)
- tb_cross_layer_ft2232h.v: AGC opcode vectors 0x28-0x2C in Exercise A (Bug #6)

Vivado 50T build verified: WNS=+0.002ns, WHS=+0.028ns — all timing constraints met.
All tests pass: MCU 21/21, GUI 120/120, cross-layer 29/29, FPGA 25/25 (68 checks).

9_Firmware/9_3_GUI/radar_dashboard.py

@@ -734,10 +734,10 @@ class RadarDashboard:
         mode_str = "AUTO" if status.agc_enable else "MANUAL"
         mode_color = GREEN if status.agc_enable else FG
         self._agc_badge.config(text=f"AGC: {mode_str}", foreground=mode_color)
-        self._agc_current_gain_lbl.config(
-            text=f"Current Gain: {status.agc_current_gain}")
-        self._agc_current_peak_lbl.config(
-            text=f"Peak Mag: {status.agc_peak_magnitude}")
+        self._agc_gain_value.config(
+            text=f"Gain: {status.agc_current_gain}")
+        self._agc_peak_value.config(
+            text=f"Peak: {status.agc_peak_magnitude}")
 
         total_sat = sum(self._agc_sat_history)
         if total_sat > 10:
@@ -746,8 +746,8 @@ class RadarDashboard:
             sat_color = YELLOW
         else:
             sat_color = GREEN
-        self._agc_sat_total_lbl.config(
-            text=f"Total Saturations: {total_sat}", foreground=sat_color)
+        self._agc_sat_badge.config(
+            text=f"Saturation: {total_sat}", foreground=sat_color)
 
         # ---- Throttle matplotlib redraws ---------------------------------
         now = time.monotonic()