this makes a small (no-op) change to
`fmd_precision_within_grace_period`.

this will provide an event that can be used to identify why a clue did
not satisfy the consensus rules. using tracing's pretty formatter:

```
  2024-02-26T21:26:49.825156Z ERROR penumbra_app::action_handler::transaction::stateful: invalid clue precision, clue_precision: 1, using_current_precision: false, using_previous_precision: false, within_grace_pe
riod: true
    at crates/core/app/src/action_handler/transaction/stateful.rs:62
    in penumbra_app::action_handler::transaction::stateful::fmd_precision_within_grace_period with current_fmd.precision_bits: 0, previous_fmd.precision_bits: 0, previous_fmd.as_of_block_height: 1
    in penumbra_app::action_handler::transaction::check_stateful
    in penumbra_mock_consensus::abci::deliver_tx
    in penumbra_mock_consensus::block::execute with height: 1, time: 1708982809
```

fixes #3788.

This commit moves some common functionality into the MockClient itself, in
order to simplify the layout of the test logic.  Also, we add explicit
invariant checks on the chain state.