Replace SPM program-unit simplification with program-specific rules

[MaxGhenis]

Context

microunit currently uses SPM units as the temporary simplification for program-specific units. This is intentional for the first Microplex integration: it keeps the unit pipeline moving while preserving a single relationship-based unit layer.

The current simplification appears in:

• assign_program_partition_from_spm
• assign_ego_units_from_spm
• assign_snap_partition default fallback
• medicaid_magi_from_spm

Why this needs to get better

SPM units are not the correct unit definition for many programs:

• SNAP households depend on food purchase/preparation, elderly/disabled exceptions, and disqualified members.
• Medicaid MAGI households are focal-person units and can overlap within the same physical household.
• SSI is closer to individual/couple units.
• TANF assistance units vary by state.
• ACA/PTC uses tax-family and coverage-household concepts.
• Housing assistance units can follow still different household rules.

Using SPM units for all of these will likely bias program eligibility and benefit simulations, especially in multi-family households, households with unrelated members, cohabiters, and mixed tax/SPM arrangements.

Proposed direction

Build program-specific constructors on top of shared primitives:

1. A source-normalized person graph: persons, households, parent pointers, spouse/partner links, age, relationship codes, disability flags, student flags, and source-specific metadata.
2. Partition-style constructors where each person belongs to one unit, e.g. SNAP as an approximation when applicable.
3. Ego-unit constructors where each focal person gets a membership set, e.g. Medicaid MAGI.
4. Diagnostics comparing native IDs, SPM simplification, and program-specific outputs when source data provides references.
5. Microplex export helpers so microplex-us can choose between spm_simplification and true program units explicitly.

Acceptance criteria

• Keep the SPM simplification available behind an explicit flag or named helper.
• Add at least one true program-specific constructor beyond SPM/tax.
• Add tests showing cases where SPM simplification and the true program unit differ.
• Add diagnostics that report how often the true constructor differs from SPM units within households.
• Wire microplex-us to record which unit mode was used in generated artifacts.

Initial candidate scope

Start with SNAP or Medicaid MAGI:

• SNAP is likely easier as a partition-style constructor.
• Medicaid MAGI is more important for proving EgoUnitMembership because units overlap by focal person.