Add Stage 5 release candidate validator

Author: anth-volk

changelog.d/1024.added

@@ -0,0 +1 @@
+Added a Stage 5 release candidate validation service backed by the shared validation core.

docs/engineering/stages/release_promotion.md

@@ -55,6 +55,13 @@ or step manifests. Release-specific details such as missing staged artifacts,
 missing validation reports, finalized-release conflicts, version mismatches, or
 destination conflicts should live in canonical finding metadata.
 
+Use `ReleaseCandidateValidator` for the `5a_validate_outputs` library seam.
+It wraps `policyengine_us_data.validation_core` and calls existing release
+guards through injected dependencies, including staged-artifact presence,
+release-manifest preflight, matching finalized-manifest checks, and release
+completion marker checks. Keep those dependencies injectable so unit tests do
+not need Hugging Face, GCS, Modal, or production credentials.
+
 ## Rerun Comparison Material
 
 Before public writes, rerun and reuse decisions should compare semantic

policyengine_us_data/release_promotion/__init__.py

@@ -23,15 +23,27 @@
 )
 from .context import ReleasePromotionContext
 from .validation import build_release_candidate_shape_report
+from .validation import (
+    DEFAULT_REQUIRED_RELEASE_ARTIFACT_FAMILIES,
+    RELEASE_VALIDATION_SUBSTAGE_ID,
+    ReleaseCandidateValidationDependencies,
+    ReleaseCandidateValidator,
+    default_release_candidate_validation_dependencies,
+)
 
 __all__ = [
     "BASE_RELEASE_ARTIFACT_PATHS",
+    "DEFAULT_REQUIRED_RELEASE_ARTIFACT_FAMILIES",
+    "RELEASE_VALIDATION_SUBSTAGE_ID",
     "ReleaseArtifactSpec",
     "ReleaseCandidateInputBundle",
     "ReleasePromotionContext",
+    "ReleaseCandidateValidationDependencies",
+    "ReleaseCandidateValidator",
     "build_legacy_release_candidate_bundle",
     "build_release_candidate_bundle_from_stage4_contract",
     "build_release_candidate_shape_report",
+    "default_release_candidate_validation_dependencies",
     "dedupe_normalized_release_paths",
     "infer_artifact_identity",
     "infer_release_artifact_spec",