Slice text by Unicode scalar position

A slice of a string returns Result[str, AllocError], with the same clamping rules as lists. Positions count Unicode scalar values, just like len and indexing:

def main() -> Result[(), AllocError | IoError]:
    text = "Aé🙂Z"
    print(str.repr(text[1:-1])?)?
    print(str.repr(text[-100:100])?)?
    print(str.repr(text[3:1])?)?
    print(str.repr(text[::-1])?)?
    print(str.repr(text.slice(1, -1))?)?
    print(text)?
    Ok(())

Result[str, AllocError].Ok("é🙂")
Result[str, AllocError].Ok("Aé🙂Z")
Result[str, AllocError].Ok("")
Result[str, AllocError].Ok("Z🙂éA")
Result[str, AllocError].Ok("é🙂")
Aé🙂Z

text.slice(start, stop) is the method form, with required bounds and no step. The result owns its UTF-8 bytes and outlives the source. The operation creates only the final string, including for an empty or whole-string slice, so even these cases can return Err(AllocError.OutOfMemory). Source strings are never modified. Combining marks count separately; positions are not grapheme clusters or byte offsets.