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/tests/cross_layer/contract_parser.py

@@ -108,12 +108,23 @@ class ConcatWidth:
 
 def parse_python_opcodes(filepath: Path | None = None) -> dict[int, OpcodeEntry]:
     """Parse the Opcode enum from radar_protocol.py.
-    Returns {opcode_value: OpcodeEntry}.
+    Returns {opcode_value: OpcodeEntry}, excluding MCU_ONLY_OPCODES.
+    MCU-only opcodes have no FPGA case statement and must not appear in
+    the bidirectional Python/Verilog contract check.
     """
     if filepath is None:
         filepath = GUI_DIR / "radar_protocol.py"
     text = filepath.read_text()
 
+    # Extract MCU_ONLY_OPCODES set so we can exclude those values below.
+    mcu_only: set[int] = set()
+    m_set = re.search(r'MCU_ONLY_OPCODES[^=]*=\s*frozenset\(\{([^}]*)\}\)', text)
+    if m_set:
+        for tok in m_set.group(1).split(','):
+            tok = tok.strip()
+            if tok.startswith(('0x', '0X')):
+                mcu_only.add(int(tok, 16))
+
     # Find the Opcode class body
     match = re.search(r'class Opcode\b.*?(?=\nclass |\Z)', text, re.DOTALL)
     if not match:
@@ -123,7 +134,8 @@ def parse_python_opcodes(filepath: Path | None = None) -> dict[int, OpcodeEntry]
     for m in re.finditer(r'(\w+)\s*=\s*(0x[0-9a-fA-F]+)', match.group()):
         name = m.group(1)
         value = int(m.group(2), 16)
-        opcodes[value] = OpcodeEntry(name=name, value=value)
+        if value not in mcu_only:
+            opcodes[value] = OpcodeEntry(name=name, value=value)
     return opcodes