Assign SNAP take-up from reported receipt and the FNS participation rate

[daphnehanse11]

Summary

Closes #243. Second half of the SNAP work-requirements input surface (#248), alongside #293 (hours).

The published dataset ships takes_up_snap_if_eligible constant True (or omits it — same engine default), so PolicyEngine-US pays SNAP to 100% of eligible units. FNS measures participation among the eligible at ~82%. Universal take-up misstates who receives SNAP and overstates the reach of any eligibility-side reform.

This adds a snap_take_up source stage following the #266/#293 template: manifest entry + shared runtime handler + frame transform + release gate + builder wiring.

Semantics (identical to the retired enhanced-CPS pipeline)

1. Reported recipients always take up — SPM units with positive raw ASEC SPM_SNAPSUB reported receiving SNAP; survey measurement wins unconditionally.
2. Non-reporters fill to the published rate — seeded draws grant take-up at exactly the fill rate needed for the overall weighted share to land on the manifest's cited FNS rate (0.82). When reporters alone exceed the rate, nothing is forced — the share is emergent.
3. Eligibility stays the engine's job — the flag is assigned across all units and PE-US intersects it with modeled eligibility, matching the retired pipeline.

Draws are blake2b hashes keyed by stable source identity (source_year/source_household_id/min source_person_id), so support-channel clones of one source unit always agree and reruns are bit-reproducible. The rate lives in the manifest with its FNS citation — a rate without a citation refuses to run.

Healing + gate

• The frame transform is idempotent on a signal-carrying column and recomputes when the column is constant — the published all-True landmine is repaired, not trusted.
• Release gate (snap_take_up_signal): column nonconstant, every reporting unit takes up (anchor preserved), weighted share within [0.70, 0.95]. Threaded through release gate failures, calibration diagnostics, and both manifests, mirroring the immigration and hours gates.

Verification

• 18 new tests (manifest declaration, anchor/fill/overflow semantics, seed stability, clone consistency, citation enforcement, healing path, gate failure modes); full populace-build suite green.
• End-to-end against the current 233,716-SPM-unit base: take-up share 0.821 (target 0.82), reported share 0.107 (CPS underreporting is exactly why the fill exists), zero reporters denied, gate green.

Follow-up (not in this PR)

State-calibrated take-up (greedy fill against FNS state household counts, the calibrate_binary_assignment machinery) is deliberately deferred: FNS state counts are average-monthly while the flag is annual-ever, and that bridge deserves its own reviewed decision — see the discussion on #292. National-rate + reported-anchor matches the retired pipeline's published behavior.

Merge-order notes

• Same anchor-point conflicts with Derive hours-worked inputs from ASEC reported hours #293 (both insert after the immigration stage in the manifest, stage order, and builder) — trivial resolution whichever lands second.
• After this merges, the takes_up_snap_if_eligible reviewed exclusion in Gate releases on input columns stuck at the engine default #286's degenerate-input gate becomes stale and should be removed.

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