Clean up code reuse, consistency, and efficiency issues

Merge readExistingSerial and readExistingContent into a single
readExisting function to eliminate duplicate file I/O. Extract dateBase
helper to deduplicate serial formula between defaultSerial and
bumpSerial. Cache hash results during collision check to avoid
recomputing per member. Normalize error prefixes (remove "error:" from
fmt.Errorf, add uniformly at print sites). Use filepath.Join instead of
manual "/" concatenation. Replace trivial containsStr wrapper with
strings.Contains. Simplify tokenize to a single return. Use writeTestFile
and fixedTime helpers consistently in tests.

main.go

@@ -41,7 +41,7 @@ func main() {
 
 	members, err := parseInput(inputFile, cfg)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "%s\n", err)
+		fmt.Fprintf(os.Stderr, "error: %s\n", err)
 		os.Exit(1)
 	}
 
@@ -58,15 +58,15 @@ func main() {
 	for _, catName := range catNames {
 		changed, err := processCatalog(catName, cfg, members[catName], *outputDir, now)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "%s\n", err)
+			fmt.Fprintf(os.Stderr, "error: %s\n", err)
 			hasErrors = true
 			continue
 		}
-		catZone := cfg.Catalogs[catName].Zone
+		zoneFile := cfg.Catalogs[catName].Zone + "zone"
 		if changed {
-			fmt.Fprintf(os.Stderr, "%s%s: updated\n", catZone, "zone")
+			fmt.Fprintf(os.Stderr, "%s: updated\n", zoneFile)
 		} else {
-			fmt.Fprintf(os.Stderr, "%s%s: unchanged\n", catZone, "zone")
+			fmt.Fprintf(os.Stderr, "%s: unchanged\n", zoneFile)
 		}
 	}