fix(mcu): MCU-A5 — gate Idq health-window during PA calibration walk

The boot-time Idq calibration walks DAC_val from 126 down toward the
1.680 A target. Mid-walk readings sit well above the 2.5 A overcurrent
threshold by design, and a channel that hits the safety_counter timeout
(50 iters) can be left above the window. Without a gate, the next
checkSystemHealth() pass would trip ERROR_RF_PA_OVERCURRENT and route
straight into Emergency_Stop, killing the system mid-bringup.

Added a `pa_calibration_in_progress` flag set TRUE around both DAC1 and
DAC2 cal walks. checkSystemHealth's Idq window short-circuits while the
flag is set; bias-fault and overcurrent thresholds remain fully active
once the walk completes, so any genuinely stuck-high channel surfaces on
the very next health pass and routes through the normal handler.

Other health checks (lock, comm, temperature, watchdog) stay live during
cal — no behavioural change to anything except the Idq window.

Added test_mcu_a5_pa_cal_gate (7 cases): mid-walk masking, post-cal
re-arming, stuck-high channel surfacing after gate clears, bias-fault
gating, PowerAmplifier=false short-circuit, and a pre-fix regression
case showing the buggy path would have tripped overcurrent mid-walk.
MCU regression now 79/79.

9_Firmware/9_1_Microcontroller/tests/Makefile

@@ -68,6 +68,7 @@ TESTS_STANDALONE := test_bug12_pa_cal_loop_inverted \
                     test_bug16_runradar_shadows_globals \
                     test_mcu_a1_cooling_hysteresis \
                     test_mcu_a7_emergency_persist \
+                    test_mcu_a5_pa_cal_gate \
                     test_gap3_iwdg_config \
                     test_gap3_temperature_max \
                     test_gap3_idq_periodic_reread \
@@ -167,6 +168,9 @@ test_mcu_a1_cooling_hysteresis: test_mcu_a1_cooling_hysteresis.c
 test_mcu_a7_emergency_persist: test_mcu_a7_emergency_persist.c
 	$(CC) $(CFLAGS) $< -o $@
 
+test_mcu_a5_pa_cal_gate: test_mcu_a5_pa_cal_gate.c
+	$(CC) $(CFLAGS) $< -o $@
+
 # Gap-3 safety tests -- mock-only (needs spy log for GPIO sequence)
 test_gap3_emergency_stop_rails: test_gap3_emergency_stop_rails.c $(MOCK_OBJS)
 	$(CC) $(CFLAGS) $(INCLUDES) $< $(MOCK_OBJS) -o $@