feat: AGC phases 4-5 — STM32 outer-loop AGC class + main.cpp integration

Implements the STM32 outer-loop AGC (ADAR1000_AGC) that reads the FPGA
saturation flag on DIG_5/PD13 once per radar frame and adjusts the
ADAR1000 VGA common gain across all 16 RX channels.

Phase 4 — ADAR1000_AGC class (new files):
- ADAR1000_AGC.h/.cpp: attack/recovery/holdoff logic, per-channel
  calibration offsets, effectiveGain() with OOB safety
- test_agc_outer_loop.cpp: 13 tests covering saturation, holdoff,
  recovery, clamping, calibration, SPI spy, reset, mixed sequences

Phase 5 — main.cpp integration:
- Added #include and global outerAgc instance
- AGC update+applyGain call between runRadarPulseSequence() and
  HAL_IWDG_Refresh() in main loop

Build system & shim fixes:
- Makefile: added CXX/CXXFLAGS, C++ object rules, TESTS_WITH_CXX in
  ALL_TESTS (21 total tests)
- stm32_hal_mock.h: const uint8_t* for HAL_UART_Transmit (C++ compat),
  __NOP() macro for host builds
- shims/main.h + real main.h: FPGA_DIG5_SAT pin defines

All tests passing: MCU 21/21, GUI 92/92, cross-layer 29/29.

9_Firmware/9_1_Microcontroller/tests/stm32_hal_mock.h

@@ -34,6 +34,10 @@ typedef uint32_t HAL_StatusTypeDef;
 
 #define HAL_MAX_DELAY 0xFFFFFFFFU
 
+#ifndef __NOP
+#define __NOP() ((void)0)
+#endif
+
 /* ========================= GPIO Types ============================ */
 
 typedef struct {
@@ -182,7 +186,7 @@ GPIO_PinState HAL_GPIO_ReadPin(GPIO_TypeDef *GPIOx, uint16_t GPIO_Pin);
 void          HAL_GPIO_TogglePin(GPIO_TypeDef *GPIOx, uint16_t GPIO_Pin);
 uint32_t      HAL_GetTick(void);
 void          HAL_Delay(uint32_t Delay);
-HAL_StatusTypeDef HAL_UART_Transmit(UART_HandleTypeDef *huart, uint8_t *pData, uint16_t Size, uint32_t Timeout);
+HAL_StatusTypeDef HAL_UART_Transmit(UART_HandleTypeDef *huart, const uint8_t *pData, uint16_t Size, uint32_t Timeout);
 
 /* ========================= SPI stubs ============================== */