Impute US humanitarian immigration statuses to cited stocks (H.R.1 channels) - #779

Fixes #767.

Implements the design proposed and detailed in the issue thread: the US immigration_status source stage now draws humanitarian/temporary-protection statuses to externally cited stocks, so the enacted H.R.1 eligibility channels stop computing ≈$0 on the next build.

What changes

Kernel (us_runtime/immigration.py). Between the ASEC-UA legal-status indicators and the EAD worker/student spill, the stage runs thirteen seeded draws — sequential, mutually exclusive, in decreasing order of target precision:

• PAROLED_ONE_YEAR per program origin: OAW Afghans (73,566), U4U Ukrainians (158,000), CHNV Nicaraguans (93,070) and Venezuelans (117,330);
• REFUGEE (160k ≈ FY2023+FY2024 OHSS admissions; refugees must apply for LPR adjustment after one year) and ASYLEE (160k ≈ FY2022–24 OHSS grants; 88% of 2015–22 affirmative adult grantees had adjusted by end-2024) from the indicator-documented pool only;
• TPS per country from the CRS RS20844 table (Venezuela 605,015; El Salvador 170,125 with the 2001 continuous-residence cutoff binding at PEINUSYR granularity; Honduras/Nicaragua/Nepal/other-designated likewise);
• DEPORTATION_WITHHELD carries an explicit cited zero (EOIR grants ~2.2k/year and publishes no stock — below the CPS's resolution for a defensible draw).

Cuba/Haiti-born are excluded everywhere (the CUBAN_HAITIAN_ENTRANT class is the statutorily favorable label H.R.1 preserves — Haiti TPS rides it), as is the DACA statutory cohort; Ukraine/Afghanistan TPS registrants are carried by the parole draw (same U4U/OAW people). Residual-pool draws (parole/TPS only) flip ssn_card_type NONE → NON_CITIZEN_VALID_EAD, preserving the NONE ⇔ UNDOCUMENTED gate invariant, and the spill controls still bind the remaining residual pool, so the Pew undocumented-worker/student margins land exactly as before.

Manifest & spec. Every target lives in source_stages.json with one citation per number (all URLs verified live, several by exact byte match against the quoted documents); the sources schema gains the required humanitarian_status_stocks block; the bundle regenerates; thirteen _stable_site declarations join the seed protocol with owner-ledger bindings, and the BE/UK envelope digests re-pin because the shared seed-protocol source is attested.

Gates & probes. The composition gate bands each category's emitted mass against its cited target (explicit-zero categories must emit zero) and records achieved-vs-target in details for certification notes. Four release-blocking reform-coverage probes pin the channels end-to-end — restoring the pre-H.R.1 Medicaid list (§71109, 2027), the below-FPL PTC exception (§71302, 2026), the ACA lawful-presence list (§71301, 2027), and the pre-§10108 SNAP list (2026) must each move their budget measure on the export, binding only through immigration_status_str.

Evidence

Running the extended stage on the raw 2024 ASEC (seed 0): REFUGEE 162k (1.02× target, 74% reporting Medicaid), ASYLEE 160k (1.00×), PAROLED_ONE_YEAR 405k (0.92× — the Ukraine pool saturates below its admin count because the ASEC undercovers 2022–24 arrivals; recorded, in-band), TPS 865k (1.01×), undocumented 12.97M (1.18× the Pew anchor, in-band), undocumented workers 8.14M against the 8.3M control. CHIP-age humanitarian mass is present (TPS under-19: 120k weighted).

test_us_hr1_immigration_channels.py proves each probe's channel on synthetic households against the locked engine (medicaid +$10.6k, aca_ptc +$7.7k/+$6.3k, snap +$2.6k per household, all sign-correct from $0 baselines) — the PR-CI early warning for the engine side without restricted microdata.

Deliberately out of scope (filed)

• US: store years_since_us_entry from PEINUSYR (blocked on engine ICHIA option) #776 — store years_since_us_entry (blocked on the engine's CHIPRA-214/ICHIA option; the below-FPL repeal channel binds via TPS meanwhile).
• Verification/redetermination churn flag surface (option 2 of #767, generalized) #777 — verification/redetermination churn flag surface (the surviving generalization of the issue's option 2; needs engine consumers first).
• Add the CHIP §71109 reform-coverage probe when the engine encodes it (pe-us#9346) #778 — CHIP probe once policyengine-us#9346 lands (engine 1.819.0 tests only ~UNDOCUMENTED for CHIP).

No certified release is touched; the imputation activates in the next build lane, where the smoke gate enforces the four channels permanently.

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