Add ACA bronze premium companion variables

[daphnehanse11]

Closes #8145

Summary

• add lcbp, lcbp_age_0, lcbp_age_curve_amount_person, and lcbp_family_tier_amount as bronze companions to the existing ACA silver variables
• add a 2026 rating-area parameter table for lowest-cost bronze premiums in the same age-0 monthly format as SLCSP
• add focused bronze ACA tests, docs, and changelog entry

Data notes

• derives 2026 bronze premiums from the live KFF calculator buckets using the representative ZIP mapping we already use for ACA rating areas
• FL rating area 4 uses ZIP 32042 instead of missing 32026; both are in Bradford County and resolve to the same bronze premium
• NE rating area 1 uses ZIP 68005 instead of missing 68056; both are in Sarpy County and resolve to the same bronze premium

Testing

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/aca/lcbp -c policyengine_us

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (20705b0) to head (e58f1ff).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8155       +/-   ##
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+ Coverage   85.36%   100.00%   +14.63%     
============================================
  Files           3         8        +5     
  Lines          41       122       +81     
  Branches        2         0        -2     
============================================
+ Hits           35       122       +87     
+ Misses          6         0        -6     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+14.63%)	⬆️

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[MaxGhenis]

Fix for gross LCBP premium eligibility is pushed and CI is green.