fix(mcu): add FAULT_ACK command to clear system_emergency_state via USB (closes #83)

The volatile fix in the companion PR (#118) makes the safe-mode blink loop
escapable in principle, but no firmware path existed to actually clear
system_emergency_state at runtime — hardware reset was the only exit, which
fires the IWDG and re-energises the PA rails that Emergency_Stop() cut.

This change adds a FAULT_ACK command (opcode 0x40): the host sends an exact
4-byte CDC packet [0x40, 0x00, 0x00, 0x00]; USBHandler detects it regardless
of USB state and sets fault_ack_received; the blink loop checks the flag each
250 ms iteration and clears system_emergency_state, allowing a controlled
operator-acknowledged recovery without triggering a watchdog reset.

Detection is guarded to exact 4-byte packets only. Scanning larger packets
for the subsequence would false-trigger on the IEEE 754 big-endian encoding
of 2.0 (0x4000000000000000), which starts with the same 4 bytes and can
appear in normal settings doubles.

FAULT_ACK is excluded from the FPGA opcode enum to preserve the
Python/Verilog bidirectional contract test; contract_parser.py reads the
new MCU_ONLY_OPCODES frozenset in radar_protocol.py to filter it.

7 new test vectors in test_gap3_fault_ack_clears_emergency.c cover:
detection, loop exit, loop hold without ack, settings false-positive
immunity, truncated packet, wrong opcode, and multi-iteration sequence.

Reported-by: shaun0927 (Junghwan) <https://github.com/shaun0927>

9_Firmware/9_1_Microcontroller/tests/Makefile

@@ -70,7 +70,8 @@ TESTS_STANDALONE := test_bug12_pa_cal_loop_inverted \
                     test_gap3_idq_periodic_reread \
                     test_gap3_emergency_state_ordering \
                     test_gap3_overtemp_emergency_stop \
-                    test_gap3_health_watchdog_cold_start
+                    test_gap3_health_watchdog_cold_start \
+                    test_gap3_fault_ack_clears_emergency
 
 # Tests that need platform_noos_stm32.o + mocks
 TESTS_WITH_PLATFORM := test_bug11_platform_spi_transmit_only
@@ -178,6 +179,9 @@ test_gap3_overtemp_emergency_stop: test_gap3_overtemp_emergency_stop.c
 test_gap3_health_watchdog_cold_start: test_gap3_health_watchdog_cold_start.c
 	$(CC) $(CFLAGS) $< -o $@
 
+test_gap3_fault_ack_clears_emergency: test_gap3_fault_ack_clears_emergency.c
+	$(CC) $(CFLAGS) $< -o $@
+
 # Tests that need platform_noos_stm32.o + mocks
 $(TESTS_WITH_PLATFORM): %: %.c $(MOCK_OBJS) $(PLATFORM_OBJ)
 	$(CC) $(CFLAGS) $(INCLUDES) $< $(MOCK_OBJS) $(PLATFORM_OBJ) -o $@