Swap the microcosm-built UK spine into the national build and retire the incumbent feed

[juaristi22]

Parent: #145 (workstream E of the UK-migration epic #665). Successor to #686. Reconciled 2026-08-25 with the #623 calibration-seam plan after the first armed calibration campaign (9 runs on the uk-first-calibrated-run-623-686 build branch).

Scope, and what moved out

This issue owns the spine lane after #747 merges: the defect batch the calibration campaign surfaced (A), the per-step gating regime for the spine build (B), and the W9–W11 retirement plus dead code/markdown cleanup (C).

Moved out to #623 (PR #743): the calibration-only driver. The new production calibration seam (tools/calibrate_uk_national_dataset.py + calibration_seam.py) is the "national driver becomes calibration-only" step — it takes whichever h5 it is given, materializes measures, solves weights under doctrine, and never modifies a data variable. What stays here is the other half: retiring the legacy June path (tools/build_uk_national_dataset.py, with its SPI re-derivation and imputation stages) once the seam is proven, which is spine-lane cleanup per the 2026-08-24 ruling.

Gate: nothing here starts until #747 is reviewed and merged. Every item either modifies the spine build (invalidating #747's licensed evidence) or retires a fallback the comparison depends on.

State at the point of handover

leg	state
L0 build + determinism	complete — 3 rungs, twins payload-identical, record identity exact at 52,846
L1 identity receipts	e4–e8 green on both the post-E8 and pre-E8 spines
L2 whole-spine parity	signed_parity — 26 beyond-band divergences all signed, 0 unsigned, --strict clean
L3 baselines	measured — 177 input-mass totals, 47 QRF tail grids
First armed calibration	loss 0.079 · 378/388 bindable · candidate beats incumbent 293–48 on the frozen register · fiscal totals (UC, income tax) still 10–18% low

A. Spine defect remainder

Two of the five landed in #747 rather than waiting for a rebuild here — the review made the case that fixing the spine before the swap is cheaper than certifying it twice:

• 12 structural-NaN SPI person columns fixed in Whole-spine parity for the UK: re-pinned incumbent, comparison instruments, signed register, and the E10 evidence (#686) #747: the SPI stage now writes the adjudicated stage-time zero on the FRS channel, so the artifact loads and the calibration seam's NaN fence can stay fail-loud without refusing every spine artifact. This no longer gates the first production calibration.
• 85+ age-tail disaggregation fixed in Whole-spine parity for the UK: re-pinned incumbent, comparison instruments, signed register, and the E10 evidence (#686) #747 as the age_tail declarative source stage, with the six ONS band populations committed as a resource whose every cell records the register target id it must agree with, and the campaign's person_source_id clone-twin keying preserved.

Still open here:

1. Spine sidecar self-pin. spine-c.build.json carries the full stage census, artifact/resource pins, seeds and rules-engine version — and no reader joins it today. Downstream consumers should inherit provenance from the sidecar rather than re-deriving it; wire the join and pin the h5 to its sidecar.
2. Support-channel gaps: slc.repayments.england_postgraduate has zero support on the spine; the top-income (£1m+) channel holds 2 records on both datasets. Each needs a measured support channel or an explicit adjudication that the gap is accepted.
3. Investigation, not yet a code fix: fiscal totals 10–18% low on the calibrated candidate increasingly looks like spine income distribution rather than target defects. Assessment lane; findings feed back here or into a successor.

B. Per-step gating regime for the spine build

The 2026-08-24 ruling: no imputation or spine gating in the calibration path — the seam runs a calibration-scoped battery only. That is only coherent if the spine build polices itself, stage by stage, the way the US builds do (#327 doctrine):

6. Per-stage imputation-health gates. Each imputation stage gets a gate consuming its own receipts (support bounds, incidence vs donor, conservation) so a regression is caught at the stage that caused it, not at a whole-spine diff. The signed-differences register stays the swap-decision record; the per-step gates are the standing health checks.

7. Wire the dormant CGT donor support bounds — or decide they cannot be terminal. cgt_band_donor_support_bounds.json is declared in country_package.json and read by nothing; the other four support-bounds resources (was_wealth, lcfs_consumption, etb_vat, etb_services) are each listed in the uk_support gate's support_bounds_resources and in battery_bindings.py. The CGT one is not an oversight — the resource records why in its own semantic_fit_note: its intervals describe donor-stage initial support, and E8's hmrc_cgt_gains_spine Table-3 redraw moves amounts onto a different band surface afterwards, so "terminal support-gate applicability requires reviewer confirmation".

   That makes this the sharpest argument for per-step gating rather than a stray wiring chore. The other four work as terminal gates because nothing rewrites their columns after they clip to donor; CGT is the one stage where a terminal gate provably cannot express the constraint. Two ways out, and the choice is the adjudication: gate the donor stage at stage time, before the redraw, on the bounds as they stand; or derive post-redraw bounds from the Table-3 surface and gate terminally like the others. Either way the resource stops being declared-but-dead. (This also closes the third of the three sign-offs left open on Port the CGT structure, salary sacrifice, and student loan stages onto the UK spine at FRS 2024-25 #740.)

8. Re-home the family-coverage contract. uk_release_family_build_stages requires 12 required_at_build families; the spine driver builds 10, the June driver 2, so neither driver alone can pass — the W9 seam. Fix by pointing the guard at the spine plan that actually builds the families (the spine's hmrc_spi_income_spine / hmrc_cgt_gains_spine supersede the June driver's two). Never by relaxing the gate, and never by sidecar plumbing — family_coverage status='required_at_build' is an executable contract, not documentation.

9. Arm the thresholds against the spine's own line (US Export input-mass-parity gate references the raw base, flagging correct calibration gains on PUF-imputed columns #327: a parity gate compares a release to its own previous certified line, not a foreign artifact): flip uk_input_mass_parity's reference to the spine's first certified release; arm uk_qrf_tail_concentration from the Assemble the microcosm-built UK spine: whole-spine parity and swap acceptance #686 L3 baselines (47 QRF tail grids, 12 household); RC battery + macro_realism on the swap build. Every armed threshold records which run measured it.

10. Release-cut certification. The microcosm-data release-verification contract expects the full 21-entry battery on a published artifact — that expectation maps to the monolith. Combining spine-side per-step gating + the seam's calibration-scoped battery into a publishable certification is contract evolution; until it lands, seam-built records say shippable: false with the reason.

C. W9–W11 retirement, dead code, markdowns

11. Retire the legacy June path once the seam reproduces the campaign result (the v9-equivalent live proof in the Calibrate the UK national build from Ledger-backed targets #623 plan): tools/build_uk_national_dataset.py's SPI re-derivation, replace_uk_spi_support_tables's zero-weight-channel refusal, and the CGT redraw all encode the June convention the spine has superseded.
12. W8 — certified-identity registry entry for the spine artifact. W11 — retire the CGT injection path the incumbent needed.
13. UK_CANDIDATE_DATASET_NAME → microcosm_uk_2024 (uk_runtime/terminal_gates.py:72, currently populace_uk_2023). Ruling 2026-08-25: the candidate carries the microcosm name, not the legacy populace one — superseding the earlier populace_uk_2024 note; the repo itself has moved to PolicyEngine/microcosm, and the parity-instrument tests already use microcosm_uk_2024.h5 as the canonical candidate filename. Sweep the stray populace_uk_2024* fixture names in test_uk_rowwise_build_driver.py in the same pass. Check UK_REFERENCE_DATASET_NAME alongside: once the spine is the build, "reference" means the previous certified microcosm line, not the enhanced-FRS artifact.
14. Un-pin the incumbent from microcosm-data/contract.py:219 and uk_runtime/weighted_integrity.py:241 once nothing reads it as an input. The parity reference itself stays — it is the historical comparison instrument and keeping it is how the swap remains auditable.
15. Dead code: UKPolicyEngineAdapter (zero production callers — the harness rebuilt its capability outboard); the assessment harness tools/calibrate_uk_national_spine.py retires once the seam driver covers it.
16. Markdown dispositions — a committed .md is a claim nobody re-checks; promote to a gate/test, make regenerable, or archive with a date:

file	disposition
UK_COVERAGE_PROGRESS.md	stale and misleading — baselined on the enhanced-FRS 2023-24 artifact; live content already lives in release_input_coverage_manifest.json. Delete.
experiments/686-uk-spine-comparison-ledger.md	the adjudication packet; make regenerable so it cannot drift from the register beside it
experiments/686-uk-spine-swap-receipts.md	R0–R5 run receipts. Archive as dated history once the swap lands
experiments/630-uk-gate-adjudication-receipts.md	check each threshold it justifies is pinned in gates.json with provenance, then archive
experiments/612-uk-carrier-payload-receipt.md	the "should be a test" case — confirm compare_uk_h5_payload.py covers it, then delete
CONTRACT_FINDINGS.md	#462 findings; the fix is merged and gated. Delete

Sweep the same pattern outside the UK lane (COVERAGE_PROGRESS.md, PROGRESS_BUILD*.md, _698-*.md, _*LANE-NOTES.md, FINAL_REPORT.md) as a separate cross-lane PR.

D. Acceptance

• A: the spine self-pins its sidecar and resolves or adjudicates both support-channel gaps. (The NaN and age-tail fixes landed in Whole-spine parity for the UK: re-pinned incumbent, comparison instruments, signed register, and the E10 evidence (#686) #747; its licensed evidence and parity receipt are re-measured there.)
• B: every imputation stage gated on its own receipts, including the CGT donor stage, whose support bounds are either wired at stage time or re-derived post-redraw rather than left declared-but-dead; the family-coverage guard passes against the spine plan unchanged; uk_input_mass_parity references the spine's own certified line and uk_qrf_tail_concentration is armed from L3, each with run provenance.
• C: the national build runs from the microcosm spine with no policyengine-uk-data artifact as an input; the June path and the CGT injection are retired; the candidate is microcosm_uk_2024; no committed Markdown asserts an invariant not also enforced by a test or gate.
• The whole-spine parity instrument still runs green against the frozen reference, as the historical record of what the swap changed.

Carried forward, not blocking

• owned_land reviewed exclusion expires 2026-09-20 — calendar-critical, independent of this issue.
• would_claim_uc frozen at 0.55 (U8; uk-data#452) — the pre-registered lever if an armed run's UC fit is strained.
• The incumbent's ETB per-head division — a second upstream defect of the UK: banded CGT gains targets (by size of gain) to constrain the distribution, not just count + total #467 family, observed at Assemble the microcosm-built UK spine: whole-spine parity and swap acceptance #686, not filed. Needs a decision.
• Calibration-lane follow-ups (welfare-cap UC fact, multi-family concept gap, UC nil-award/top-band cells, degenerate composition bindings) live with Calibrate the UK national build from Ledger-backed targets #623, not here.