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.

helpers_test.go

@@ -24,7 +24,3 @@ func assertContains(t *testing.T, s, substr string) {
 		t.Errorf("expected %q to contain %q", s, substr)
 	}
 }
-
-func containsStr(s, substr string) bool {
-	return strings.Contains(s, substr)
-}