c-26 stacked pool: emergent undocumented population 18.9M vs the 11.0M anchor (1.72x; gate band [0.5, 1.6])

Found by the candidate-26 dense-evidence arm (release id populace-us-2024-onesurface-pkg3-legacy-dense-scoring-b8819b3f-20260902T184743Z, base pool populace-us-2024-stacked-f025-s578-asec42213-acs382903-20260831T162338Z-e14b24e8, builder at 5ef9237e). The pre-export immigration composition gate (us_immigration_composition_gate, invoked from tools/build_us_fiscal_refresh_release.py after the pool's immigration inputs load) failed:

Immigration composition failed: emergent undocumented population 18,919,043 is 1.72x the published anchor 11,000,000 (band [0.5, 1.6], https://www.pewresearch.org/short-reads/2024/07/22/what-we-know-about...)

Context, stated carefully:

• This is, as far as the run records show, the first time this gate has been exercised against a stacked pool through --base-h5 (pool_frame is not None, so immigration inputs come from the pool itself rather than being derived in-build). Whether 1.72x is a c-26 regression, a general property of the stacked assembly at design weights, or a gate whose anchor comparison needs an origin-aware basis on stacked pools is exactly the open question.
• The c-26 pool is already gate_failed on the by-origin battery (63/135; #781, #783 hold the adjudication context). This failure is additional evidence for the candidate-26 ruling, not a substitute for it.
• Related but distinct: #767 (humanitarian statuses absent from the imputation) concerns statuses computing $0, not the undocumented total.

This issue is the adjudicated owner for the failure pattern Immigration composition failed: in the dense-evidence arm's --evidence-failure-owners file (microcosm#506 evidence-tier contract), so the evidence build can mint its scoring artifact with the failure recorded in known_failures rather than dying before calibration. Owning it here means: whoever closes the c-26 ruling either explains this number on the stacked basis or fixes the assembly.

🤖 Generated with Claude Code

Start with tools/build_us_fiscal_refresh_release.py and the us_immigration_composition_gate invocation after immigration inputs load. Compare the --base-h5 stacked path, where pool_frame is not None, with the published anchor and the gate band, using the c-26 release and pool records named in the issue. Review #781 and #783 for adjudication context; done means explaining the stacked-basis result or fixing the assembly and recording the failure appropriately.