feat: implement RMASTER-WP-0023 private-by-default exposure

Add the exposure contract, additive family schema fields, validator
checks and fixtures, the reef-railiance exception snapshot, and
routed intakes. Enforcement stays in the owning repos.

tools/testdata/family-declarations/good-exposure/rapp-example/declarations/rapp.yaml

@@ -0,0 +1,42 @@
+kind: managed-workload-package
+repo_family: rapp
+rapp_id: rapp-example
+repo: rapp-example
+ownership_repo: railiance-platform
+contract_version: 1.0.0
+readiness_state: declared
+workload_identity:
+  name: example
+  package_type: helm-managed-platform-service
+data_classification: internal
+criticality: medium
+primary_rail: rail-kubernetes
+supported_rails:
+  - rail-kubernetes
+bound_reefs:
+  - reef-example
+runtime_dependencies:
+  - kubernetes-api
+composition:
+  purpose: Fixture rapp used to exercise exposure validation.
+  member_repos:
+    - repo: rapp-example
+      role: package
+      deployables:
+        - example
+rollout_contract:
+  default_mode: helm-upgrade-install
+smoke_contract:
+  required:
+    - example-status-ok
+rollback_contract:
+  order:
+    - helm-rollback
+exposure:
+  posture: public
+  binding_admission: production-approved
+  grant:
+    hostname: example.coulomb.social
+    reason: fixture public rapp
+    approved_on: "2026-08-15"
+    residual_risk_owner: railiance-platform