Shellcode encoders seem broken

The `encode()` routine should not ever emit bytes in `avoid`.

```py
>>> encode(asm(shellcraft.sh()), avoid='\x01')
b'jhh///sh/bin\x89\xe3h\x01\x01\x01\x01\x814$ri\x01\x011\xc9Qj\x04Y\x01\xe1Q\x89\xe11\xd2j\x0bX\xcd\x80'
```

Start by locating the encode() routine and reproduce the documented asm(shellcraft.sh()) example with avoid='\x01'. Trace how the encoder handles forbidden bytes, then verify that the output contains none of the requested avoid bytes and that existing encoder behavior remains intact.

Written by the indexing model from the issue text.