`ModelOutputThunk` should not be a `CBlock`

[nrfulton]

Justification: Python pattern matching is based on order not specificity, so the following code never makes it into the second branch.

match span:
  case CBlock():
    handle cblocks generically
  case ModelOutputThunk():
    handle mots in particular
  case Component():
    handle more complex stuff

This is a footgun. The easiest solution is to simply remove mot from the cblock type hierarchy. There are no disadvanatges: anywhere we expect a cblock or a mot we should be explicit about that with a union type anyways, because mot.value is semantically different from cb.value

[jakelorocco]

@nrfulton, this is probably yet another design question that we should revisit.

[jakelorocco]

Do we still want to make this change @nrfulton?

[AngeloDanducci]

cc @nrfulton @jakelorocco did we reach a definite design choice on this?

[jakelorocco]

Discussed: We should make ModelOutputThunks a separate class.