Add evidence context policy preparation

internal/framework/pipeline/evidence_policy.go

@@ -0,0 +1,20 @@
+package pipeline
+
+// EvidenceContextPolicy controls optional source-context publication by an
+// output encoder.
+type EvidenceContextPolicy struct {
+	Enabled     bool
+	WindowUnits int
+	LaneIDs     []string
+}
+
+// EvidenceContextPolicyProvider is implemented by output encoders that opt in
+// to evidence-context publication.
+type EvidenceContextPolicyProvider interface {
+	EvidenceContextPolicy() EvidenceContextPolicy
+}
+
+func cloneEvidenceContextPolicy(policy EvidenceContextPolicy) EvidenceContextPolicy {
+	policy.LaneIDs = append([]string(nil), policy.LaneIDs...)
+	return policy
+}