docs(tvix/TODO): add Value::Json TODO

Change-Id: Id47c1e3f98cb0bf53006b002417b69cfa49b7410
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12721
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Ilan Joselevich <personal@ilanjoselevich.com>

tvix/docs/src/TODO.md

@@ -10,6 +10,15 @@ Feel free to add new ideas. Before picking something, ask in `#tvix-dev` to make
 sure noone is working on this, or has some specific design in mind already.
 
 ## Cleanups
+### Evaluator
+ - There's not really a good reason why the `tvix_eval::Value::Json` enum kind
+   exists.
+  `builtins.toJSON` should simply produce a string with context, and everything
+   else should be a hidden implementation detail and should not be leaked to
+   `Value`.
+   This is a hack, as we wanted to use `serde_json` as is, but should be cleaned
+   up.
+
 ### Nix language test suite
  - Think about how to merge, but "categorize" `tvix_tests` in `glue` and `eval`.
    We currently only have this split as they need a different feature set /