fix: correct interlace detection and reap idet child

The previous detector substring-matched "TFF"/"BFF" against idet's own
label text, so it returned true on every source, and `cmd.Start()` was
never paired with `Wait()`, leaving a zombie ffmpeg per file.

Run idet bounded with `-frames:v 400 -an -sn -f null -` so it completes,
use CombinedOutput so the child is reaped, and parse the "Multi frame
detection" summary line — interlaced only when TFF+BFF > Progressive.

internal/encoder/encoder.go

@@ -6,12 +6,15 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"strconv"
 	"strings"
 
 	"videnc-vibe/pkg/types"
 )
 
+var idetSummaryRegex = regexp.MustCompile(`Multi frame detection:\s+TFF:\s*(\d+)\s+BFF:\s*(\d+)\s+Progressive:\s*(\d+)\s+Undetermined:\s*(\d+)`)
+
 type Encoder struct {
 	ffmpegPath  string
 	ffprobePath string
@@ -122,20 +125,43 @@ func (e *Encoder) GetMediaInfo(path string) (width, height int, interlaced bool,
 		}
 	}
 
-	cmd = exec.Command(e.ffmpegPath, "-i", path, "-hide_banner", "-vf", "idet")
-	stderr, _ := cmd.StderrPipe()
-	cmd.Start()
-	buf := make([]byte, 16384)
-	n, _ := stderr.Read(buf)
-	stderr.Close()
-	outputStr := string(buf[:n])
-	interlaced = strings.Contains(outputStr, "TFF") || strings.Contains(outputStr, "BFF")
+	cmd = exec.Command(e.ffmpegPath,
+		"-hide_banner",
+		"-nostats",
+		"-i", path,
+		"-vf", "idet",
+		"-frames:v", "400",
+		"-an", "-sn",
+		"-f", "null", "-",
+	)
+	idetOut, _ := cmd.CombinedOutput()
+	interlaced = detectInterlaced(string(idetOut))
 
 	fmt.Printf("DEBUG interlaced: %v\n", interlaced)
 
 	return width, height, interlaced, nil
 }
 
+// detectInterlaced parses ffmpeg idet's "Multi frame detection" summary line
+// and returns true when TFF+BFF clearly outnumber Progressive frames among
+// the decided frames. Undetermined frames are ignored. If the summary line
+// is missing, returns false (default to progressive).
+func detectInterlaced(idetOutput string) bool {
+	m := idetSummaryRegex.FindStringSubmatch(idetOutput)
+	if m == nil {
+		return false
+	}
+	tff, _ := strconv.Atoi(m[1])
+	bff, _ := strconv.Atoi(m[2])
+	prog, _ := strconv.Atoi(m[3])
+	interlaced := tff + bff
+	decided := interlaced + prog
+	if decided == 0 {
+		return false
+	}
+	return interlaced > prog
+}
+
 func (e *Encoder) calculateZscaleWidth(path string, originalHeight int) (string, int, error) {
 	cmd := exec.Command(e.ffprobePath, "-v", "error", "-select_streams", "v:0", "-show_entries", "stream=width,height,sample_aspect_ratio", "-print_format", "json", path)
 	output, err := cmd.CombinedOutput()