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_1_Microcontroller/9_1_3_C_Cpp_Code/main.cpp

@@ -2117,8 +2117,9 @@ int main(void)
       runRadarPulseSequence();
 
       /* [AGC] Outer-loop AGC: read FPGA saturation flag (DIG_5 / PD13),
-       * adjust ADAR1000 VGA common gain once per radar frame (~258 ms). */
-      {
+       * adjust ADAR1000 VGA common gain once per radar frame (~258 ms).
+       * Only run when AGC is enabled — otherwise leave VGA gains untouched. */
+      if (outerAgc.enabled) {
           bool sat = HAL_GPIO_ReadPin(FPGA_DIG5_SAT_GPIO_Port,
                                       FPGA_DIG5_SAT_Pin) == GPIO_PIN_SET;
           outerAgc.update(sat);