Implement QuetzalWriter._generate_ifhd_chunk()

The IFhd chunk contains 13 bytes of metadata identifying the story
and current execution state:
- Release number (2 bytes) from header
- Serial number (6 bytes) from header
- Checksum (2 bytes) from header
- Program counter (3 bytes) from CPU state

This allows save files to be validated against the story file.

src/mudlib/zmachine/quetzal.py

@@ -377,15 +377,40 @@ class QuetzalWriter:
         """Return a chunk of type IFhd, containing metadata about the
         zmachine and story being played."""
 
-        ### TODO: write this.  payload must be *exactly* 13 bytes, even if
-        ### it means padding the program counter.
+        mem = self._zmachine._mem
 
-        ### Some old infocom games don't have checksums stored in header.
-        ### If not, generate it from the *original* story file memory
-        ### image and put it into this chunk.  See ZMemory.generate_checksum().
-        pass
+        # Release number (2 bytes, big-endian) from header bytes 2-3
+        release = mem.read_word(2)
 
-        return "0"
+        # Serial number (6 bytes) from header bytes 0x12-0x17
+        serial = bytes(mem[0x12:0x18])
+
+        # Checksum (2 bytes, big-endian) from header bytes 0x1C-0x1D
+        checksum = mem.read_word(0x1C)
+
+        # Program counter (3 bytes, big-endian) - current PC
+        pc = self._zmachine._cpu._program_counter
+
+        # Build the 13-byte chunk
+        chunk_data = bytearray(13)
+
+        # Bytes 0-1: Release number
+        chunk_data[0] = (release >> 8) & 0xFF
+        chunk_data[1] = release & 0xFF
+
+        # Bytes 2-7: Serial number
+        chunk_data[2:8] = serial
+
+        # Bytes 8-9: Checksum
+        chunk_data[8] = (checksum >> 8) & 0xFF
+        chunk_data[9] = checksum & 0xFF
+
+        # Bytes 10-12: Program counter (24-bit)
+        chunk_data[10] = (pc >> 16) & 0xFF
+        chunk_data[11] = (pc >> 8) & 0xFF
+        chunk_data[12] = pc & 0xFF
+
+        return bytes(chunk_data)
 
     def _generate_cmem_chunk(self):
         """Return a compressed chunk of data representing the compressed

tests/test_zmachine_opcodes.py

@@ -784,6 +784,57 @@ class ZMachineComplexOpcodeTests(TestCase):
             self.cpu.op_restart()
 
 
+class QuetzalWriterTests(TestCase):
+    """Test suite for QuetzalWriter save functionality."""
+
+    def test_generate_ifhd_chunk(self):
+        """Test _generate_ifhd_chunk() produces correct 13-byte IFhd chunk."""
+        from mudlib.zmachine.quetzal import QuetzalWriter
+
+        # Create a mock zmachine with known header values
+        mock_zmachine = Mock()
+        mock_zmachine._mem = MockMemory()
+
+        # Set header values in memory:
+        # Bytes 2-3: Release number (0x1234)
+        mock_zmachine._mem.write_word(0x02, 0x1234)
+        # Bytes 0x12-0x17: Serial number (6 bytes: "860509")
+        serial = b"860509"
+        for i, byte in enumerate(serial):
+            mock_zmachine._mem[0x12 + i] = byte
+        # Bytes 0x1C-0x1D: Checksum (0xABCD)
+        mock_zmachine._mem.write_word(0x1C, 0xABCD)
+
+        # Set program counter
+        mock_cpu = Mock()
+        mock_cpu._program_counter = 0x123456  # 24-bit PC
+        mock_zmachine._cpu = mock_cpu
+
+        # Create writer and generate chunk
+        writer = QuetzalWriter(mock_zmachine)
+        chunk_data = writer._generate_ifhd_chunk()
+
+        # Verify chunk is exactly 13 bytes
+        self.assertEqual(len(chunk_data), 13)
+
+        # Verify release number (bytes 0-1)
+        self.assertEqual(chunk_data[0], 0x12)
+        self.assertEqual(chunk_data[1], 0x34)
+
+        # Verify serial number (bytes 2-7)
+        for i, expected in enumerate(serial):
+            self.assertEqual(chunk_data[2 + i], expected)
+
+        # Verify checksum (bytes 8-9)
+        self.assertEqual(chunk_data[8], 0xAB)
+        self.assertEqual(chunk_data[9], 0xCD)
+
+        # Verify program counter (bytes 10-12)
+        self.assertEqual(chunk_data[10], 0x12)
+        self.assertEqual(chunk_data[11], 0x34)
+        self.assertEqual(chunk_data[12], 0x56)
+
+
 # Note: ZObjectParser methods are tested through integration tests
 # with real story files, not unit tests with mock memory, as the
 # interaction with ZStringFactory makes mocking complex.