Add deterministic file ordering in Builder.Build()

Sort file entries by path (lexicographic, byte-order) before
serialization to ensure deterministic output. Add fixedUUID support
for testing reproducibility, and a test asserting byte-identical
output from two runs with the same input.

Closes #23

mfer/serialize.go

@@ -49,8 +49,13 @@ func (m *manifest) generateOuter() error {
 		return errors.New("internal error")
 	}
 
-	// Generate UUID and set on inner message
-	manifestUUID := uuid.New()
+	// Use fixed UUID if provided, otherwise generate a new one
+	var manifestUUID uuid.UUID
+	if len(m.fixedUUID) == 16 {
+		copy(manifestUUID[:], m.fixedUUID)
+	} else {
+		manifestUUID = uuid.New()
+	}
 	m.pbInner.Uuid = manifestUUID[:]
 
 	innerData, err := proto.MarshalOptions{Deterministic: true}.Marshal(m.pbInner)