Add release-gate preflight: catch statically-knowable gate failures without a solve

README.md

@@ -60,6 +60,48 @@ See [SYSTEM_REQUIREMENTS.md](SYSTEM_REQUIREMENTS.md) for the measured memory,
 disk, and CPU footprint of developing and building locally (and what to budget
 on a build machine — RAM is the binding constraint).
 
+## Release-gate preflight
+
+Several release gates fail on facts that are already determined by the base
+pool, the frozen selection, and the target/coverage registry — no calibration
+solve needed to see them. `tools/preflight_us_release_gates.py` recovers those
+signals in **minutes** so a two-hour release launch is not the first place a
+knowable defect surfaces:
+
+```bash
+uv run python tools/preflight_us_release_gates.py \
+  --base-h5 out/base-m/base_populace_us_2024_puf_support.h5 \
+  --selection-source-manifest inputs/buildm_keogh_swap_selection_source.json \
+  --export-input-mass-reference-h5 forensics/populace_us_2024.h5
+```
+
+It is read-only against the H5 artifacts and reports, per check, `PASS` /
+`FAIL` / `AT-RISK` with the measured numbers (exit `1` on any FAIL, `2` on
+AT-RISK only, `0` clean):
+
+1. **Selection carryover** — the frozen selection-source manifest maps cleanly
+   onto the base pool (the frozen-support recovery contract, run pre-solve).
+2. **Zero-support preview** — compiled positive fiscal targets whose
+   materialized support is ~0 under the selection at base weights stay a
+   structural zero after the solve. Direct-column targets are checked;
+   engine-derived measures are marked *not statically checkable* (pass
+   `--ledger-facts` to compile the target surface).
+3. **Export-mass parity risk** — each export-mass column's pool mass at *base*
+   weights against its reference band, honoring the release tool's
+   `US_EXPORT_INPUT_MASS_REVIEWED_EXCLUSIONS` register (reused, never
+   re-declared). A column out of band pre-solve is flagged for review.
+4. **Smoke-probe support audit** — every reform-coverage probe leaf's pool vs
+   *selected* nonzero support and pool sign-leg decomposition. A leaf with pool
+   support but zero selected support **fails** (the input the frozen selection
+   cannot express); a thin selection or a signed leaf whose net sign
+   contradicts the probe's `expected_sign` is AT-RISK.
+
+**Run it** at base-build exit, before any release launch, and after any change
+to the selection-source manifest or the target/coverage registry. The
+synthetic-fixture unit tests
+(`packages/populace-build/tests/test_us_release_gate_preflight.py`) run in the
+normal `uv run pytest` suite; the real-H5 mode above is a local/runbook step.
+
 ## Releasing & alerts
 
 Publishing uploads the locally built `releases/<id>/` artifacts to the Hugging