Collect Medicaid eligibility from the ACA source batches instead of a second full-population pass

[daphnehanse11]

Implements the deferred efficiency finding from the #334 review: the medicaid_take_up stage re-ran a full-population batched Microsimulation sweep (~68 batches at the default 5k-household batch size) immediately after the ACA source loop tore down identical per-batch simulations, solely to read person-level is_medicaid_eligible. Eliminating the second pass saves roughly 15–45 minutes of wall clock and the repeated multi-GB simulation churn per release build.

Note

Stacked on #334 (medicaid-chip-take-up) — the function being refactored only exists there. Retarget to main once #334 merges.

What changed

• Medicaid eligibility now rides the ACA source batches. _aca_source_tax_unit_table_batched (and its wrappers up through _with_aca_marketplace_source_outputs) accept an on_batch_simulation(batch_frame, batch_simulation) callback invoked while each batch simulation is alive. The release flow passes a new _BatchedPersonEligibilityCollector, which calculates is_medicaid_eligible per batch and scatters it into a full-population person array by person_id position. _with_medicaid_take_up_outputs accepts the precomputed array and keeps the dedicated batched pass only as a fallback; the simulation is not None fast paths are preserved.
• Safety of the seam. The ACA writeback touches only takes_up_aca_if_eligible and selected_marketplace_plan_benchmark_ratio. Verified against installed policyengine-us that neither appears in is_medicaid_eligible's transitive dependency closure (468 variables, including adds/subtracts), so computing eligibility on the pre-writeback batch simulations is behavior-preserving. The collector tracks coverage and raises on partial coverage rather than shipping silent all-False rows; a never-invoked collector yields None, which triggers the fallback pass.
• Batching skeleton deduplicated. The GC-suspension / full-batch-reuse / forced-collection loop that was copied four times is now a shared _for_each_household_batch(frame, batch_size=…, describe=…, fn=…) helper.

Blast radius

Beyond the ACA and Medicaid loops, the two reform-validation batch loops were also rewritten onto the shared helper: _BatchedReformValidationSimulation._calculate_total (validation-measure scoring) and _reform_household_income_tax (JCT reform target materialization). Their per-batch bodies are unchanged — same dataset construction, reform application, cache invalidation, and household-position writeback — but they now share the skeleton, so a bug in the helper would touch all four passes.

Tests

• New: ACA batched loop feeds the collector in full person-table order; collector rejects partial coverage; _with_medicaid_take_up_outputs uses a precomputed array without triggering the batched pass (asserted via diagnostics eligible weight).
• Updated: three monkeypatched signatures that gained keyword-only params.
• Full packages/populace-build suite passes (519 tests); ruff check/format clean.

🤖 Generated with Claude Code