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/annexb_reader_test.go

@@ -0,0 +1,173 @@
+package pkg
+
+import (
+	"bytes"
+	_ "embed"
+	"math/rand"
+	"testing"
+
+	"m7s.live/v5/pkg/codec"
+	"m7s.live/v5/pkg/util"
+)
+
+func bytesFromMemory(m util.Memory) []byte {
+	if m.Size == 0 {
+		return nil
+	}
+	out := make([]byte, 0, m.Size)
+	for _, b := range m.Buffers {
+		out = append(out, b...)
+	}
+	return out
+}
+
+func TestAnnexBReader_ReadNALU_Basic(t *testing.T) {
+
+	var reader AnnexBReader
+
+	// 3 个 NALU，分别使用 4 字节、3 字节、4 字节起始码
+	expected1 := []byte{0x67, 0x42, 0x00, 0x1E}
+	expected2 := []byte{0x68, 0xCE, 0x3C, 0x80}
+	expected3 := []byte{0x65, 0x88, 0x84, 0x00}
+
+	buf := append([]byte{0x00, 0x00, 0x00, 0x01}, expected1...)
+	buf = append(buf, append([]byte{0x00, 0x00, 0x01}, expected2...)...)
+	buf = append(buf, append([]byte{0x00, 0x00, 0x00, 0x01}, expected3...)...)
+
+	reader.AppendBuffer(append(buf, codec.NALU_Delimiter2[:]...))
+
+	// 读取并校验 3 个 NALU（不包含起始码）
+	var n util.Memory
+	if err := reader.ReadNALU(nil, &n); err != nil {
+		t.Fatalf("read nalu 1: %v", err)
+	}
+	if !bytes.Equal(bytesFromMemory(n), expected1) {
+		t.Fatalf("nalu1 mismatch")
+	}
+
+	n = util.Memory{}
+	if err := reader.ReadNALU(nil, &n); err != nil {
+		t.Fatalf("read nalu 2: %v", err)
+	}
+	if !bytes.Equal(bytesFromMemory(n), expected2) {
+		t.Fatalf("nalu2 mismatch")
+	}
+
+	n = util.Memory{}
+	if err := reader.ReadNALU(nil, &n); err != nil {
+		t.Fatalf("read nalu 3: %v", err)
+	}
+	if !bytes.Equal(bytesFromMemory(n), expected3) {
+		t.Fatalf("nalu3 mismatch")
+	}
+
+	// 再读一次应无更多起始码，返回 nil 错误且长度为 0
+	if err := reader.ReadNALU(nil, &n); err != nil {
+		t.Fatalf("expected nil error when no more nalu, got: %v", err)
+	}
+	if reader.Length != 4 {
+		t.Fatalf("expected length 0 after reading all, got %d", reader.Length)
+	}
+}
+
+func TestAnnexBReader_AppendBuffer_MultiChunk_Random(t *testing.T) {
+
+	var reader AnnexBReader
+
+	rng := rand.New(rand.NewSource(1)) // 固定种子，保证可复现
+
+	// 生成随机 NALU（仅负载部分），并构造 AnnexB 数据（随机 3/4 字节起始码）
+	numNALU := 12
+	expectedPayloads := make([][]byte, 0, numNALU)
+	fullStream := make([]byte, 0, 1024)
+
+	for i := 0; i < numNALU; i++ {
+		payloadLen := 1 + rng.Intn(32)
+		payload := make([]byte, payloadLen)
+		for j := 0; j < payloadLen; j++ {
+			payload[j] = byte(rng.Intn(256))
+		}
+		expectedPayloads = append(expectedPayloads, payload)
+
+		if rng.Intn(2) == 0 {
+			fullStream = append(fullStream, 0x00, 0x00, 0x01)
+		} else {
+			fullStream = append(fullStream, 0x00, 0x00, 0x00, 0x01)
+		}
+		fullStream = append(fullStream, payload...)
+	}
+	fullStream = append(fullStream, codec.NALU_Delimiter2[:]...) // 结尾加个起始码，方便读取到最后一个 NALU
+	// 随机切割为多段并 AppendBuffer
+	for i := 0; i < len(fullStream); {
+		// 每段长度 1..7 字节（或剩余长度）
+		maxStep := 7
+		remain := len(fullStream) - i
+		step := 1 + rng.Intn(maxStep)
+		if step > remain {
+			step = remain
+		}
+		reader.AppendBuffer(fullStream[i : i+step])
+		i += step
+	}
+
+	// 依次读取并校验
+	for idx, expected := range expectedPayloads {
+		var n util.Memory
+		if err := reader.ReadNALU(nil, &n); err != nil {
+			t.Fatalf("read nalu %d: %v", idx+1, err)
+		}
+		got := bytesFromMemory(n)
+		if !bytes.Equal(got, expected) {
+			t.Fatalf("nalu %d mismatch: expected %d bytes, got %d bytes", idx+1, len(expected), len(got))
+		}
+	}
+
+	// 没有更多 NALU
+	var n util.Memory
+	if err := reader.ReadNALU(nil, &n); err != nil {
+		t.Fatalf("expected nil error when no more nalu, got: %v", err)
+	}
+}
+
+// 起始码跨越两个缓冲区的情况测试（例如 00 00 | 00 01）
+func TestAnnexBReader_StartCodeAcrossBuffers(t *testing.T) {
+	var reader AnnexBReader
+	// 构造一个 4 字节起始码被拆成两段的情况，后跟一个短 payload
+	reader.AppendBuffer([]byte{0x00, 0x00})
+	reader.AppendBuffer([]byte{0x00})
+	reader.AppendBuffer([]byte{0x01, 0x11, 0x22, 0x33}) // payload: 11 22 33
+	reader.AppendBuffer(codec.NALU_Delimiter2[:])
+	var n util.Memory
+	if err := reader.ReadNALU(nil, &n); err != nil {
+		t.Fatalf("read nalu: %v", err)
+	}
+	got := bytesFromMemory(n)
+	expected := []byte{0x11, 0x22, 0x33}
+	if !bytes.Equal(got, expected) {
+		t.Fatalf("payload mismatch: expected %v got %v", expected, got)
+	}
+}
+
+//go:embed test.h264
+var annexbH264Sample []byte
+
+var clipSizesH264 = [...]int{7823, 7157, 5137, 6268, 5958, 4573, 5661, 5589, 3917, 5207, 5347, 4111, 4755, 5199, 3761, 5014, 4981, 3736, 5075, 4889, 3739, 4701, 4655, 3471, 4086, 4428, 3309, 4388, 28, 8, 63974, 63976, 37544, 4945, 6525, 6974, 4874, 6317, 6141, 4455, 5833, 4105, 5407, 5479, 3741, 5142, 4939, 3745, 4945, 4857, 3518, 4624, 4930, 3649, 4846, 5020, 3293, 4588, 4571, 3430, 4844, 4822, 21223, 8461, 7188, 4882, 6108, 5870, 4432, 5389, 5466, 3726}
+
+func TestAnnexBReader_EmbeddedAnnexB_H265(t *testing.T) {
+	var reader AnnexBReader
+	offset := 0
+	for _, size := range clipSizesH264 {
+		reader.AppendBuffer(annexbH264Sample[offset : offset+size])
+		offset += size
+		var nalu util.Memory
+		if err := reader.ReadNALU(nil, &nalu); err != nil {
+			t.Fatalf("read nalu: %v", err)
+		} else {
+			t.Logf("read nalu: %d bytes", nalu.Size)
+			if nalu.Size > 0 {
+				tryH264Type := codec.ParseH264NALUType(nalu.Buffers[0][0])
+				t.Logf("tryH264Type: %d", tryH264Type)
+			}
+		}
+	}
+}