Lifeline categorical eligibility omits Veterans and Survivors Pension Benefit (47 CFR 54.409(a)(2))

## Summary

[47 CFR § 54.409(a)(2)](https://www.ecfr.gov/current/title-47/section-54.409#p-54.409(a)(2)) lists five programs whose participants qualify for Lifeline: Medicaid, SNAP, SSI, Federal Public Housing Assistance, and the Veterans and Survivors Pension Benefit. After #9378 (FPHA, closes #9376), `gov.fcc.lifeline.categorical_eligibility` covers four of the five. The pension route is still only a comment:

```yaml
# Also not implemented:
# - Veterans and Survivors Pension Benefit
```

Because `is_acp_eligible` reads the same Lifeline list, ACP has the same gap.

## Why it is not a one-line fix

The only veterans input is `veterans_benefits` (Person, YEAR, USD). It is the CPS ASEC `VET_VAL` total, which pools all VA payments: disability compensation, DIC, pension, education benefits, and other. There is no pension-only component in the data or the model, and `is_veteran` is defined as `veterans_benefits > 0` (see #5238).

Adding `veterans_benefits` to the list therefore treats any VA payment as the pension. The regulation qualifies only Veterans Pension and Survivors Pension recipients, which are themselves means-tested (38 U.S.C. 1521, 1541). A veteran receiving disability compensation with income above 135% FPG would be wrongly qualified.

## Options

1. **Add `veterans_benefits` as is**, with a comment documenting the over-inclusion. API partners can be told to populate it only with pension amounts for this purpose. Microsimulation aggregates are unaffected today because `phone_cost` and `broadband_cost` are never populated in policyengine-us-data, so `lifeline` is $0 for every record.
2. **Add a reported-receipt flag** for VA pension (parallel to `receives_ssi`, `receives_snap`, `receives_housing_assistance`) and put that in the list. Cleaner semantics, but it is a new input with no survey source.
3. **Leave the gap documented** and close this issue as won't-fix.

Option 1 or 2 should also be applied to `is_acp_eligible` implicitly via the shared list; no ACP code change is needed.

## Files

- `policyengine_us/parameters/gov/fcc/lifeline/categorical_eligibility.yaml`
- `policyengine_us/variables/gov/fcc/lifeline/is_lifeline_eligible.py`
- `policyengine_us/variables/gov/fcc/acp/is_acp_eligible.py`
- `policyengine_us/variables/household/income/person/veterans_benefits.py` (input, no formula)

## Related

- #9376 / #9378: FPHA route (this issue is the last missing program from the same regulation, raised as a follow-up in review there).
- #5238: consolidate `is_veteran`, which has the same root cause (`veterans_benefits` is an aggregate).
- #9389: the veteran and active-duty exception in the federal noncitizen rules needs a veteran-status signal that is likewise not derivable from `veterans_benefits`.

Start with policyengine_us/parameters/gov/fcc/lifeline/categorical_eligibility.yaml and compare the shared eligibility logic in policyengine_us/variables/gov/fcc/lifeline/is_lifeline_eligible.py and policyengine_us/variables/gov/fcc/acp/is_acp_eligible.py. Read veterans_benefits.py and the related issues to resolve whether the aggregate input is acceptable or a new receipt signal is needed. Done means the chosen pension semantics are documented and applied consistently without changing unrelated veteran eligibility.

Written by the indexing model from the issue text.