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.

input_test.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"os"
 	"path/filepath"
 	"testing"
 )
@@ -206,9 +205,7 @@ zone.example.org    catalog1, catalog2
 
 test.example.net    catalog1, group=internal
 `
-	if err := os.WriteFile(inputPath, []byte(content), 0o644); err != nil {
-		t.Fatal(err)
-	}
+	writeTestFile(t, dir, "zones.txt", content)
 
 	members, err := parseInput(inputPath, cfg)
 	if err != nil {
@@ -240,9 +237,7 @@ func TestParseInputErrors(t *testing.T) {
 	t.Run("invalid line in input", func(t *testing.T) {
 		dir := t.TempDir()
 		path := filepath.Join(dir, "zones.txt")
-		if err := os.WriteFile(path, []byte("zone-with-no-catalog\n"), 0o644); err != nil {
-			t.Fatal(err)
-		}
+		writeTestFile(t, dir, "zones.txt", "zone-with-no-catalog\n")
 		_, err := parseInput(path, cfg)
 		if err == nil {
 			t.Fatal("expected error for invalid line")