feat: add command domain invariants

internal/domain/idempotency.go

@@ -0,0 +1,27 @@
+package domain
+
+import "crypto/sha256"
+
+type RequestHash [sha256.Size]byte
+
+func HashImmutableRequest(canonical []byte) RequestHash { return sha256.Sum256(canonical) }
+
+type ReplayDecision uint8
+
+const (
+	ReplayNew ReplayDecision = iota
+	ReplayExact
+	ReplayConflict
+)
+
+// ClassifyReplay compares a canonical immutable request against durable state.
+// A nil stored hash means no mutation with this request UUID exists.
+func ClassifyReplay(stored *RequestHash, incoming RequestHash) ReplayDecision {
+	if stored == nil {
+		return ReplayNew
+	}
+	if *stored == incoming {
+		return ReplayExact
+	}
+	return ReplayConflict
+}