refactor: adopt strings.Cut and strings.SplitSeq

Applies what go fix now proposes, so make fmt is a no-op on a clean tree
instead of dirtying these two files on every run. Both rewrites are
equivalent: Cut's first result matches SplitN(s, sep, 2)[0], and SplitSeq
iterates the same substrings without building the intermediate slice.

pkg/modules/libreoffice/api/proxy_test.go

@@ -361,7 +361,7 @@ func TestSofficeProxyEnv_OverridesExisting(t *testing.T) {
 	// Old proxy values must be gone, not duplicated. Count exact-case keys.
 	counts := map[string]int{}
 	for _, kv := range out {
-		key := strings.SplitN(kv, "=", 2)[0]
+		key, _, _ := strings.Cut(kv, "=")
 		counts[key]++
 	}
 	for _, key := range []string{"http_proxy", "HTTP_PROXY", "https_proxy", "HTTPS_PROXY", "no_proxy", "NO_PROXY"} {

pkg/modules/pdfcpu/optimize.go

@@ -180,7 +180,7 @@ func (engine *PdfCpu) listImages(ctx context.Context, inputPath string) ([]pdfcp
 // their second column is not a numeric object number.
 func parseImagesList(output string) []pdfcpuImage {
 	var images []pdfcpuImage
-	for _, line := range strings.Split(output, "\n") {
+	for line := range strings.SplitSeq(output, "\n") {
 		cols := strings.Split(line, "│")
 		if len(cols) < 9 {
 			continue