TriggerX provides generic threshold-based steering for CarpetX-style runtime
scalars and arrays.

It is intended for cases where a condition on a variable or parameter should
change a scalar before CarpetX regridding runs, for example to add a BoxInBox
refinement level once a lapse threshold is crossed.

For CarpetX grid functions, TriggerX uses CarpetX's internal reduction
machinery instead of the legacy Cactus reduction handles. Supported grid
function reductions are:
- `minimum`
- `maximum`
- `sum`
- `sum_abs`
- `sum_squared`
- `average`
- `standard_deviation`
- `volume`
- `norm1`
- `norm2`
- `norm_inf`
- `maximum_abs`

Current reactions:
- `steerscalar`
- `steerparam`

The thorn runs at `postinitial` and `poststep` before `EstimateError`, so
updates to `BoxInBox::num_levels`, `BoxInBox::radius`, or similar runtime
scalars are visible to the same regridding pass.

Example:

```
ActiveThorns = "TriggerX BoxInBox"

TriggerX::trigger_number = 1
TriggerX::trigger_debug  = 1

TriggerX::trigger_checked_variable[0] = "ADMBase::alp"
TriggerX::trigger_reduction[0]        = "minimum"
TriggerX::trigger_relation[0]         = ">"
TriggerX::trigger_checked_value[0]    = 0.01

TriggerX::trigger_reaction[0]             = "steerscalar"
TriggerX::trigger_steered_scalar[0]       = "BoxInBox::num_levels[0]"
TriggerX::trigger_steered_scalar_index[0] = 0
TriggerX::trigger_steered_scalar_value[0] = "13"
```

For array targets such as `BoxInBox::radius`, use the flattened array index in
`trigger_steered_scalar_index`. For BoxInBox this is `level + 30 * region`.