Household results return numeric indices for enum variables (state_code -> 4.0, filing_status -> 0.0)

[MaxGhenis]

Found during the PR #363 review, but pre-existing and identical in the non-axes path: _safe_convert in us/household.py (and the UK counterpart) floats EnumArray int16 indices, so result.household.state_code returns 4.0 instead of "CA" and result.tax_unit.filing_status returns 0.0 instead of "SINGLE". The default person column race already ships this way. EnumArray.decode_to_str() is the proper decode. Fix belongs in the shared result-building path; needs a small sweep of existing tests that may assert the numeric form.

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