Give skill examples a county and refresh their asserted values

[anth-volk]

What and why

The skill-examples check fails on any PR right now (it surfaced on #97). The current policyengine-us (2.2.1, via policyengine 6.3.0 and spm-calculator 1.0.0) never infers SPM geography from a missing county: a household with only state_code raises

SPMInputError: County selection has no county FIPS input (''): send county_fips as a five-digit string (for example "06037"), or select geography_kind="national" in the spm configuration

so 8 of the verified examples failed. This adds a county to each of the 9 state-only households, using the same form as policyengine's own docstring ({"state_code": "NY", "county_fips": "36061"}):

State	County	FIPS
CA	Los Angeles	06037
TX	Harris	48201
NY	New York	36061

Asserted values that changed (please review)

Once the examples run, some asserted numbers no longer match. Only the ACA PTC depends on the county chosen (Anderson County, TX reproduces the old 6,250.38). The rest come out the same for any county, so they reflect model changes since this check last passed on main (2026-09-13):

Example	Value	Old	New
policyengine #0 (CA)	household_net_income	46,358	46,369
policyengine-healthcare #0 (TX)	aca_ptc	6,250.38	6,635.45 (county-dependent)
policyengine-us #1 (NY)	snap	7,364	8,061
policyengine-us #1 (NY)	household_net_income	62,681	47,929
policyengine-us #4 (CA)	ca_eitc	410.16	401.89

The prose quoting the SNAP figure is updated to match. The NY net income drop of about $15k is large enough that someone who knows the US model should confirm it is an intended model change rather than a regression. That part of this PR is the one to check.

Test plan

• pytest tests/test_skill_examples.py with policyengine[us] installed as in CI: 18 passed, 1 skipped (slow)
• uv run pytest --ignore=tests/test_skill_examples.py: 178 passed
• Confirm the NY household_net_income change (62,681 → 47,929) is expected

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