claude: bind [enc] locally to steady codec's label disambiguation

[let open Json.Codec in; map ~kind ~dec:of_string ~enc:to_string
string] made OCaml mis-resolve [string] against another [Codec.*]
path when [of_string] and [to_string] were both labeled arguments.
Pulling [enc = to_string] into a local binding before the [open]
pins the inference without any label-order sensitivity, matches
the other encodings helpers in the repo, and reads a touch better.