Declare point-in-time (average-month) semantics for enrollment take-up flags and snapshot-month provenance

Context

policyengine-us is an annual-period engine, but health program enrollment is monthly administrative reality. The build currently resolves this implicitly: the CPS anchor columns are point-in-time at interview (has_*_health_coverage_at_interview), and the cms_medicaid ledger facts are month-tagged state snapshots (e.g. month2024_12, per #321). Nothing declares what an annual *_enrolled flag means against those inputs, which invites concept drift — e.g. someone later calibrating to T-MSIS ever-enrolled-in-year counts (~10M+ higher post-unwinding) against a point-in-time anchor.

There is strong precedent for resolving this deliberately. Urban's HIPSM methodology (2020): "For Medicaid enrollment, we generally use the June enrollment report from CMS… We chose to use a point-in-time snapshot for all states compatible with our Marketplace targets, rather than an annual average." HIPSM models coverage as average-monthly/point-in-time throughout — survey anchor, admin target, and model concept all at the same instant — and handles within-year churn in separate machinery (TRIM3), not in the coverage model.

The point-in-time convention is also what keeps spending coherent: average-monthly enrollment approximates the full-year-equivalent count that CMS per-enrollee cost denominators use; ever-enrolled semantics would overstate spending unless durations were modeled.

Goal

Make the convention explicit doctrine rather than emergent behavior, so every enrollment take-up stage (#331, the ACA stage, future Medicare/BHP work) and every enrollment target shares one declared concept.

Suggested implementation

• Add a doctrine paragraph to take_up_contract.json: enrollment take-up flags represent enrollment at a representative point in time (average month), anchored on interview-point survey coverage and calibrated to month-tagged administrative snapshots; annual-flow (ever-enrolled) semantics are explicitly out of scope, deferred to the longitudinal frame direction (DESIGN.md, time-as-a-dimension).
• Require enrollment-count target facts to carry their snapshot month (already true for cms_medicaid month2024_12 rows — make it a checked convention, not an accident), and surface the month in release diagnostics/target registry output.
• Record the concept decision per target family: point-in-time snapshot vs annual average vs ever-enrolled, with the chosen concept asserted where targets are compiled.
• Feeds #230's open decision ("person-level annual coverage, interview/current coverage, or both") — coverage-universe targets should adopt the same current-coverage concept where source support allows.

Acceptance criteria

• Contract doctrine states the point-in-time convention and its rationale, citing the HIPSM precedent.
• Enrollment target facts without a declared snapshot month fail loudly or are explicitly exempted.
• #170 diagnostics and #331 gates reference the declared concept.
• Cross-links recorded: #230, #321, #331.

Start by reading take_up_contract.json and DESIGN.md, especially the time-as-a-dimension direction, then inspect the #170 diagnostics, #331 gates, and target registry references. Done means the point-in-time doctrine and HIPSM rationale are recorded, snapshot months are checked or explicitly exempted, the chosen concept is asserted per target family, and links to #230, #321, and #331 are present.