refactor: frame converter and mp4 track improvements

- Refactor frame converter implementation
- Update mp4 track to use ICodex
- General refactoring and code improvements

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

pkg/util/buffer_test.go

@@ -24,7 +24,7 @@ func TestReadBytesTo(t *testing.T) {
 		s := RandomString(100)
 		t.Logf("s:%s", s)
 		var m Memory
-		m.AppendOne([]byte(s))
+		m.PushOne([]byte(s))
 		r := m.NewReader()
 		seededRand := rand.New(rand.NewSource(time.Now().UnixNano()))
 		var total []byte
@@ -34,7 +34,7 @@ func TestReadBytesTo(t *testing.T) {
 				continue
 			}
 			buf := make([]byte, i)
-			n := r.ReadBytesTo(buf)
+			n, _ := r.Read(buf)
 			t.Logf("n:%d buf:%s", n, string(buf))
 			total = append(total, buf[:n]...)
 			if n == 0 {