Add a Populace US poverty/SPM diagnostic gate for resource distribution and negative-income tails

[MaxGhenis]

Problem

Populace needs a US validation diagnostic for the SPM/resource distribution failure modes we are investigating. These should be diagnostics, not calibration hard targets.

The diagnostic should report:

• SPM poverty rate, population count, and subgroup rates from the Populace artifact under current PolicyEngine-US.
• Resource distribution diagnostics: median/quantiles of SPM resources and relevant subgroup cuts.
• Negative-resource diagnostics: minimum SPM-unit resources, count/share with negative resources, total negative mass, and largest contributing variables where available.
• Validation metadata for official/public SPM references and raw CPS reproduction checks where feasible.
• Target residual context from source-backed hard-target families once the source coverage work lands.

Guardrails

• Do not make Census CPS SPM, DINA, or ACS poverty rates hard calibration targets.
• Treat this as a release validation diagnostic/gate; the artifact should publish the numbers and reference identity.
• Do not persist computed SPM outputs as inputs just to pass diagnostics.

Acceptance criteria

• A reproducible command/test can produce a structured poverty_spm_diagnostics.json or equivalent release artifact.
• The diagnostic includes version/reference metadata for PolicyEngine-US, the Populace artifact, and validation references.
• Tests cover negative-resource summary math, validation-only classification, and a small fixture with known poverty/resource results.
• Release manifest or diagnostics index links to the poverty/SPM diagnostic artifact.