Deduplicate Nebraska AABD program catalog surface

`policyengine_us/programs.yaml` currently exposes Nebraska AABD twice with the same variable and parameter prefix:

1. As an `ssi_state_supplement` subprogram:
- `state: NE`
- `name: Nebraska AABD`
- `variable: ne_aabd`
- `parameter_prefix: gov.states.ne.dhhs.aabd`

2. As a standalone top-level program:
- `id: ne_aabd`
- `name: Nebraska AABD`
- `variable: ne_aabd`
- `parameter_prefix: gov.states.ne.dhhs.aabd`

This showed up while wiring Axiom's PE program-surface coverage manifest: the catalog has 183 rows but only 182 distinct variables, with `ne_aabd` as the duplicate.

I think one of these should be canonicalized or the metadata should make the intentional two-surface relationship explicit; otherwise downstream parity/coverage tooling treats the same PE variable as two separate program surfaces.

Start in policyengine_us/programs.yaml and compare the two Nebraska AABD entries, including their variable and parameter_prefix fields. Confirm how the Axiom program-surface manifest counts them; done means ne_aabd is represented as one canonical surface or the intentional relationship is explicit so downstream coverage reports no duplicate variable.

Written by the indexing model from the issue text.