`sim.calc("household_income_decile", map_to="tax_unit").unique()` returns non-integer values

See [this notebook](https://colab.research.google.com/drive/15-7gAozQ9-gH-YvGpjAWYCVVb_-NJwd6#scrollTo=cIz_kXelfPuK):
```python
from policyengine_us import Microsimulation

sim = Microsimulation()

sim.calc("household_income_decile").unique()
```
returns ints from 1 to 10 (plus -1 for negative income) as expected, but:
```python
sim.calc("household_income_decile", map_to="tax_unit").unique()
```
returns non-integer values between 1 and 10.

I would have expected it to just project the household attribute down to the tax unit. Does it instead divide by people and then aggregate to tax units? Should we define it as an enum instead if we want it projected? @nikhilwoodruff

Reproduce the two `Microsimulation.calc` calls from the linked notebook and compare their unique values. Trace the definition of `household_income_decile` and the `map_to="tax_unit"` behavior to determine whether the value should be projected or aggregated; done means the tax-unit result follows the intended integer decile semantics and the behavior is covered by a regression test.

Written by the indexing model from the issue text.