perf(video): 消除中间帧复制并优化 MJPEG 有界预取

通过 Bytes 所有权和只读 AVBufferRef 引用池化 NV12 输入，消除中间 AVFrame 的整帧复制，并保留 V4L2 编码包脱离硬件缓冲的处理。

允许输出健康时有界预取 MJPEG，在输出背压时优先解码前丢帧；增加等帧、输出等待、丢帧原因及编码分段耗时统计，提供预取和所有权输入的对照开关。

补充缓冲寿命、只读保护、软件编码回退和队列策略测试及接口说明，保留现有 PTS、解码 worker、音频和加密行为。

libs/hwcodec/cpp/ffmpeg_ram/owned_nv12_frame.h

@@ -0,0 +1,51 @@
+#ifndef OWNED_NV12_FRAME_H
+#define OWNED_NV12_FRAME_H
+
+extern "C" {
+#include <libavutil/frame.h>
+}
+
+#include <limits>
+#include <memory>
+
+using RamInputRelease = void (*)(void *, uint8_t *);
+
+struct RamFrameDeleter {
+  void operator()(AVFrame *frame) const { av_frame_free(&frame); }
+};
+
+using RamOwnedFrame = std::unique_ptr<AVFrame, RamFrameDeleter>;
+
+static RamOwnedFrame make_owned_nv12_frame(const uint8_t *data, int length,
+                                            int width, int height, void *owner,
+                                            RamInputRelease release) {
+  const int64_t pixels = static_cast<int64_t>(width) * height;
+  if (!data || !release || width <= 0 || height <= 0 || width % 2 ||
+      height % 2 || pixels > std::numeric_limits<int>::max() / 3 * 2 ||
+      length < pixels + pixels / 2) {
+    if (release)
+      release(owner, const_cast<uint8_t *>(data));
+    return RamOwnedFrame();
+  }
+  RamOwnedFrame frame(av_frame_alloc());
+  if (!frame) {
+    release(owner, const_cast<uint8_t *>(data));
+    return frame;
+  }
+  frame->buf[0] = av_buffer_create(const_cast<uint8_t *>(data), length, release,
+                                  owner, AV_BUFFER_FLAG_READONLY);
+  if (!frame->buf[0]) {
+    release(owner, const_cast<uint8_t *>(data));
+    return RamOwnedFrame();
+  }
+  frame->format = AV_PIX_FMT_NV12;
+  frame->width = width;
+  frame->height = height;
+  frame->data[0] = frame->buf[0]->data;
+  frame->data[1] = frame->data[0] + pixels;
+  frame->linesize[0] = width;
+  frame->linesize[1] = width;
+  return frame;
+}
+
+#endif