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_test.go

@@ -6,7 +6,6 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
-	"time"
 )
 
 func TestIntegrationEndToEnd(t *testing.T) {
@@ -213,7 +212,7 @@ soa:
 	// Find PTR lines and verify order
 	var ptrZones []string
 	for _, line := range lines {
-		if containsStr(line, "\tPTR\t") && !containsStr(line, "coo.") {
+		if strings.Contains(line, "\tPTR\t") && !strings.Contains(line, "coo.") {
 			// Extract the PTR target
 			parts := strings.Split(line, "\t")
 			ptrZones = append(ptrZones, parts[len(parts)-1])
@@ -313,9 +312,7 @@ soa:
 		if err == nil {
 			t.Error("expected error for unknown catalog")
 		}
-		if !containsStr(string(out), "unknown catalog") {
-			t.Errorf("expected 'unknown catalog' in error output, got: %s", out)
-		}
+		assertContains(t, string(out), "unknown catalog")
 	})
 }
 
@@ -405,7 +402,7 @@ soa:
 	assertContains(t, content2, "2026011601")
 
 	// Simulate not using the tool for a while, running on a much later date
-	day3 := time.Date(2026, 6, 15, 0, 0, 0, 0, time.UTC)
+	day3 := fixedTime(2026, 6, 15)
 	members3 := []ZoneEntry{
 		{Zone: "a.example.com.", Catalogs: []string{"cat1"}, File: "test", Line: 1},
 		{Zone: "b.example.com.", Catalogs: []string{"cat1"}, File: "test", Line: 2},