Fix step() to use AdvanceInstructions command

The Step command was removed in protocol cleanup, but step() still
referenced it. Using AdvanceInstructions (0x71) which is the correct
command for stepping.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/protocol/client.ts

@@ -512,7 +512,7 @@ export class ViceClient {
     const body = Buffer.alloc(3);
     body[0] = stepOver ? 1 : 0;
     body.writeUInt16LE(count, 1);
-    const response = await this.sendCommand(Command.Step, body);
+    const response = await this.sendCommand(Command.AdvanceInstructions, body);
     this.state.running = false;
     return response;
   }