Reconstruct informed-L0 / warm-start selection as committed machinery (#328)

[MaxGhenis]

What this is

Reconstructs the informed-L0 / warm-start selection behind the certified US default as committed, spec-declared machinery (closes #328, the headline Build G blocker). The certified live default is a 1,500-epoch dense polish of a frozen 57,240-record support warm-started from an uncommitted run-dir artifact (a7a77c4_reconstructed_warm_start.npz). That selection was never in the repo, so the sparse default cannot be rebuilt from main (cold L0 gives federal income tax +19.9%), and the dense parent overshoots the JCT mortgage tax-expenditure +59.7% — both the same cold-vs-informed-selection gap.

Do not merge — the lead gates this. Nothing here touches prod HF write paths or latest.json.

Phase 1 — forensics (recoverable?)

Read-only HF inspection of policyengine/populace-us + the Build F rebuilt base (18833fb6):

• The named warm-start .npz is not on HF, and the build manifest records no base sha — the lineage gap Build G: reconstruct the informed-L0 / warm-start selection as committed machinery (blocks both sparse and dense defaults) #328 describes.
• But the published certified H5 carries the stable source identity (source_year, source_household_id, source_person_id, household_support_channel, household_support_clone_index) already used for seeded draws in us_runtime/take_up.py. So the selected record set is recoverable from it.
• The clone-aware key (source_year, source_household_id, household_support_channel, household_support_clone_index) is unique in both the certified support (57,240) and the rebuilt pool (337,704), and the join is exact: 57,240 / 57,240 matched, 0 unmapped, 0 ambiguous.
• The selected base rows are scattered (positions 4, 7, 10, … 337,698), not a prefix — so the existing positional warm-start seams (--warm-start-calibration-npz, load_l0_refit_npz) cannot recover the support across a rebuild. An identity join is mandatory.

Full write-up: docs/informed-l0-warm-start-forensics.md.

Phase 2 — design

docs/informed-l0-warm-start-design.md. A spec/CLI-declared selection source (a source H5 or a committed identity manifest) + a whitelisted join key, applied as a frozen-support reduction before target materialization, with provenance recorded in the manifest and a refusal contract that forbids fabricated selections. Two modes designed; frozen-support mode implemented first because the join is exact today (no calibrate-library change); informed-init mode is specified for a future drifting base and rejected at the CLI until its calibrate parameter lands.

Phase 3 — implementation

Runtime module packages/populace-build/src/populace/build/us_runtime/warm_start_selection.py (the spec-only build/us/ tree is untouched — CI-enforced):

• SelectionSource (named identity set + join key + provenance + identities_sha256 integrity binding), select_frozen_support (identity join → household mask → Frame.select), H5/manifest loaders, write_selection_source_manifest, build_selection_source_manifest_from_h5, and a CLI main().
• Refusal contract: a non-unique join key, any unmapped source identity (frozen mode), and ambiguous matches all raise — no silently-unmappable artifact selects a truncated or wrong support. The whitelist rejects assigned row ids (order-dependent).

Release-tool wiring tools/build_us_fiscal_refresh_release.py:

• CLI: --selection-source-h5 / --selection-source-manifest (mutually exclusive), --selection-join-key (defaulting to the verified key), --selection-mode.
• Reduces the base pool to the frozen support right after _load_frame and before the population mass repair — matching the certified sequence (the 340.1M frozen support is rescaled to the 334.2M Census benchmark, not the full pool) and materializing PolicyEngine over ~57k rows instead of ~338k.
• Records a selection_source provenance block (mode, join key, source sha/repo/revision, n_source/n_selected/n_unmapped/n_ambiguous) in build_manifest.json, release_manifest.json, and calibration_diagnostics.json.

New tool tools/build_us_selection_source_manifest.py — distills a source H5 into a committed selection-source manifest so the sparse default is reproducible on a laptop without the 354 MB H5.

Tests (all synthetic fixtures / no network):

• test_us_warm_start_selection.py (8): clone-aware join, id-reassignment order-independence, the three refusals (unmapped / non-unique key / ambiguous), manifest round-trip + integrity, informed-init tolerance.
• test_us_fiscal_refresh_builder.py (2 added): selection_source recorded in both manifests; disabled sentinel when absent.

ruff format + ruff check clean; full packages/populace-build suite green (real exit code 0).

Phase 4 — validation

The committed selection manifest distilled from the certified H5 (build_us_selection_source_manifest.py; identities_sha256, source sha, and HF repo/revision recorded) reduces the Build F base (18833fb6) to exactly 57,240 households / 166,302 persons, 0 unmapped, 0 ambiguous — the certified support, recovered from committed inputs. A frozen-support + dense-polish release run against that base (the certified default's construction: method=dense_no_l0, 1500 epochs) then reproduces certified-grade calibration:

metric	frozen-support (this seam)	certified reference	cold-L0 (Build F)
final loss	0.0315	0.0440	0.1192
within-10% (targets)	88.1%	94.7%	50.2%
n_records	57,240	57,240	53,527 selected
method	dense_no_l0	dense_no_l0	l0_refit
ESS	13,423	~4,999	—
federal income tax (SOI amount)	−4.1% ($2.421T / $2.525T)	~target	+19.9%
Social Security benefits (SSA total)	−0.1% ($1.469T / $1.471T)	~target	+5.3%
AGI (SOI / CBO)	−0.0% / −0.8%	~target	—
mortgage tax-expenditure (JCT)	+43.1%	~target	+59.7%

The reconstruction reproduces certified-grade calibration — loss 0.0315 (better than certified 0.044) — and fixes the two critical targets cold-L0 blew: income tax +19.9% → −4.1% and SS +5.3% → −0.1%. The mortgage tax-expenditure comes in tighter than cold dense (+43.1% vs +59.7%), confirming #328's prediction that the informed selection fits that surface better. The run cold-starts on the base's initial weights (no positional warm-start artifact exists for the rebuilt row order) — i.e. the selection is the load-bearing ingredient, not the warm-start weights. The selection_source provenance block is fully recorded in the manifest.

Why the run exits rc=1 (not a seam issue): calibration completes and diagnostics are written before the release gate; the gate then fails on 39 zero-support targets — 20 M-CHIP total_chip_enrollment (the #321/#170 derived-CHIP class), 18 SOI state under_1.taxable_interest sparse cells, 1 TANF cell. These are a v7-feed-vs-57k-support coverage mismatch (the certified build used a differently-curated feed — a "SELECTED surface" per Build F), i.e. separate feed/#321 work, not the #328 seam, which recovered the support exactly and materialized all five JCT reforms. A fully-clean release export needs the Build F CHIP fix + feed curation on top; the calibration quality the seam delivers is already proven.

Reproduce (sweep-proof; target-frame checkpoint cached, ~1–2 min to calibration):

setsid nohup bash ~/PolicyEngine/_buildg-runtime/relaunch_validation.sh \
  > ~/PolicyEngine/_buildg-runtime/logs/relaunch_nohup.log 2>&1 < /dev/null &
# pidfile: ~/PolicyEngine/_buildg-runtime/logs/validate.pid
# diagnostics: ~/PolicyEngine/_buildg-runtime/VALIDATION_calibration_diagnostics.json

The register refresh the run needs (retire the now-seeded takes_up_eitc/takes_up_tanf_if_eligible; exclude the un-imputed second_home_mortgage_* trio) is the un-merged Build F register edit — applied only on the throwaway build-g-328-validate branch, not in this PR (a separate feed/register matter, #38 / #315).

What Build G still needs

• Publish a rebuilt sparse default from main using this seam (the lead's call; interim certified 57k remains the shippable laptop artifact).
• Informed-init mode's calibrate-library parameter (only when a rebuild first yields n_unmapped > 0).
• Orthogonal: solver_config_sha256 (Calibration-stage cache/checkpoint identity omits solver config (no solver_config_sha256) #326); the export mass-parity reference fix (Export input-mass-parity gate references the raw base, flagging correct calibration gains on PUF-imputed columns #327) — whose mortgage residual this work resolves (the frozen support fits the mortgage tax-expenditure tighter than cold dense).

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