Indices of mappings in storage updates should refer to the post state.

[The timing of the indices in the LHS of updates](https://github.com/argotorg/act/blob/9938807e117f235585af290009fecdd95045ffbc/src/Act/Syntax/TypedExplicit.hs#L72C49-L72C55) should refer to the post state.

Start at src/Act/Syntax/TypedExplicit.hs around the linked line and trace how indices in the left-hand side of storage updates are typed and timed. Confirm how the post state is represented in this syntax, then verify that mapping indices resolve against that state; done means the update semantics consistently refer to the post state.

Written by the indexing model from the issue text.