Align MP PUF aggregate top tail with eCPS Forbes-backed branch

[MaxGhenis]

Context

While implementing #141, we verified that latest eCPS does more than the original PUF aggregate disaggregation from PolicyEngine/policyengine-us-data#627. Current upstream policyengine-us-data also includes PolicyEngine/policyengine-us-data#825, which routes the $100M+ aggregate PUF row (RECID=999999) through a Forbes/SCF-backed top-tail synthesis path before falling back to donor synthesis.

MP #141 now handles the core aggregate-row issue: it replaces MARS=0, RECID 999996-999999 aggregate rows with 122 weighted synthetic records instead of dropping them. This matches #627 at the support-count and aggregate-preservation level, but it does not yet implement eCPS #825’s Forbes-backed branch for RECID=999999.

MP already has a separate forbes_fixed_spine facility, but the active Gate-1 CPS/PUF/SIPP/SCF no-ACS run is not using it, and it is not equivalent to eCPS’s PUF-aggregate top-tail branch.

Desired outcome

Make MP’s top-tail handling production-ready for MP-vs-eCPS replacement by choosing one clear architecture and validating loss impact:

1. Either port/adapt eCPS’s Forbes-backed RECID=999999 aggregate synthesis into MP’s PUF loader, preserving MP source manifests and diagnostics; or
2. Wire MP’s existing Forbes fixed-spine path into the eCPS-shaped build in a way that is intentionally equivalent or better for the same target surface; and
3. Record the choice in the architecture catalog and artifact sidecars so this gap cannot recur silently.

Acceptance checks

• A fixture-level test proves RECID=999999 uses the chosen Forbes/top-tail path or records an explicit fallback.
• Real raw PUF smoke records the aggregate rows, synthetic row count, and whether Forbes top-tail was used.
• The eCPS-shaped Gate-1 artifact comparison includes latest local eCPS as the primary MP-vs-eCPS loss baseline and published HF eCPS as diagnostic only.
• If top-tail integration is not enabled by default, the artifact sidecar must say why.

References

• Disaggregate PUF aggregate records and fix QRF high-income training policyengine-us-data#627: Disaggregate PUF aggregate records and fix QRF high-income training
• Add Forbes-backed PUF top tail policyengine-us-data#825: Add Forbes-backed PUF top tail
• Add PUF support clone for eCPS replacement #141: Add PUF support clone for eCPS replacement