Add step() method to ZCpu for async MUD integration

Extracts the run loop body into step() which executes one instruction
and returns True/False for continue/halt. run() now delegates to step().

src/mudlib/zmachine/zcpu.py

@@ -168,31 +168,34 @@ class ZCpu:
                 log(f"Jump to offset {branch_offset:+d}")
                 self._opdecoder.program_counter += branch_offset - 2
 
+    def step(self):
+        """Execute a single instruction. Returns True if execution should continue."""
+        current_pc = self._opdecoder.program_counter
+        log(f"Reading next opcode at address {current_pc:x}")
+        (opcode_class, opcode_number, operands) = self._opdecoder.get_next_instruction()
+        implemented, func = self._get_handler(opcode_class, opcode_number)
+        log_disasm(
+            current_pc,
+            zopdecoder.OPCODE_STRINGS[opcode_class],
+            opcode_number,
+            func.__name__,
+            ", ".join([str(x) for x in operands]),
+        )
+        if not implemented:
+            log(f"Unimplemented opcode {func.__name__}, halting execution")
+            return False
+
+        # The returned function is unbound, so we must pass
+        # self to it ourselves.
+        func(self, *operands)
+        return True
+
     def run(self):
         """The Magic Function that takes little bits and bytes, twirls
         them around, and brings the magic to your screen!"""
         log("Execution started")
-        while True:
+        while self.step():
-            current_pc = self._opdecoder.program_counter
+            pass
-            log(f"Reading next opcode at address {current_pc:x}")
-            (opcode_class, opcode_number, operands) = (
-                self._opdecoder.get_next_instruction()
-            )
-            implemented, func = self._get_handler(opcode_class, opcode_number)
-            log_disasm(
-                current_pc,
-                zopdecoder.OPCODE_STRINGS[opcode_class],
-                opcode_number,
-                func.__name__,
-                ", ".join([str(x) for x in operands]),
-            )
-            if not implemented:
-                log(f"Unimplemented opcode {func.__name__}, halting execution")
-                break
-
-            # The returned function is unbound, so we must pass
-            # self to it ourselves.
-            func(self, *operands)
 
     ##
     ## Opcode implementation functions start here.