User-set ETERNITY inputs are dropped after Simulation._invalidate_all_caches (3.24.0+)

[juaristi22]

Symptom

When a Microsimulation is constructed from a dataset that supplies an ETERNITY-defined variable as input, calculate/calculate_dataframe returns the variable's default value for every row instead of the dataset-provided values, as soon as anything triggers Simulation._invalidate_all_caches (e.g. apply_reform, subsample).

We hit this in policyengine-us-data after upgrading to policyengine-us 1.674.1 / policyengine-core 3.25.3. is_household_head (a bool ETERNITY variable on Person) came back all-False from acs.calculate_dataframe([\"is_household_head\", ...]), even though the ACS H5 dataset has it correctly populated. Filtering by is_household_head then yielded zero rows and downstream code crashed.

Suspected mechanism

In policyengine_core/simulations/simulation.py:

• Holder.set_input records (variable_name, branch_name, period) into simulation._user_input_keys using the user-supplied period (e.g. Period(2024) from a dataset's time_period).
• For an ETERNITY-defined variable, Holder._set canonicalizes storage to period=ETERNITY, so _memory_storage._arrays is keyed by \"default:eternity\".
• _invalidate_all_caches then iterates _user_input_keys and looks up f\"{branch_name}:{period}\" (\"default:2024\"), which misses the canonicalized \"default:eternity\" entry.
• The preserve-loop therefore doesn't re-add the array, the holder's _arrays is wiped at the iteration over population._holders, and subsequent calculate calls fall back to the variable's default_value.

Affected versions: introduced sometime in 3.24.x (the _invalidate_all_caches + _user_input_keys machinery is in 3.24.0/3.24.4/3.25.x; not present in 3.23.6).

Minimal repro sketch

```python
import numpy as np
from policyengine_core.data import Dataset
from policyengine_core.simulations import Simulation

... a tiny tax-benefit system with one ETERNITY bool variable `flag` ...

class Tiny(Dataset):
data_format = Dataset.ARRAYS
time_period = 2024 # any non-ETERNITY period
def generate(self):
# Write {"flag": [True, False], "person_id": [1, 2], ...}
...

sim = Simulation(dataset=Tiny, tax_benefit_system=tbs)
print(sim.calculate("flag")) # [True, False]
sim.apply_reform(some_reform) # triggers _invalidate_all_caches
print(sim.calculate("flag")) # [False, False] ← bug
```

Suggested fix directions

In `_user_input_keys` accounting (in `Holder.set_input` and `Simulation.set_input`), normalize the recorded `period` to `ETERNITY` whenever `variable.definition_period == ETERNITY`, so the preserve-loop's `f"{branch_name}:{period}"` lookup matches the canonicalized storage key. Same normalization in the preserve-loop's lookup would also work.

Impact

Any country package that loads ETERNITY variables from a `Dataset` whose `time_period != ETERNITY` and that subsequently calls `apply_reform` or `subsample` will silently see those inputs replaced with default values. In `policyengine-us-data`, this took down the CPS data build's rent imputation step.