libavformat: cuframes_packets demuxer для encoded packet ring (cuframes v0.2)

Companion к существующему cuframes://. URL `cuframes_packets://<key>`
открывает encoded packet ring у publisher'а (поднятого с --enable-packet-ring)
и выдаёт raw H.264/H.265 stream без декодирования.

Use case в Frigate config:
  cameras:
    cam_parking:
      ffmpeg:
        inputs:
          - path: cuframes://cam-parking          # decoded NV12 для detect
            roles: [detect]
          - path: cuframes_packets://cam-parking  # encoded H.264 для record
            input_args: -f cuframes_packets
            roles: [record]

  → 1 RTSP connection к камере (вместо 2-3).

Implementation:
- read_header: cuframes_subscriber_create + enable_packets + get_codec_params
  (codec_id + extradata → AVStream.codecpar).
- read_packet: cuframes_subscriber_next_packet → memcpy в AVPacket.
  PACKET_OVERRUN → EAGAIN (library resync'нет на next call).
- AV_PKT_FLAG_KEY/CORRUPT/DISCONTINUITY mapping из cuframes flags.
- pts/dts: ns from publisher → µs timebase.

Build wiring:
- configure: cuframes_packets_demuxer_deps="libcuframes"
- libavformat/Makefile: CONFIG_CUFRAMES_PACKETS_DEMUXER → cuframes_packetsdec.o
- libavformat/allformats.c: extern ff_cuframes_packets_demuxer

libavformat/Makefile

@@ -173,6 +173,7 @@ OBJS-$(CONFIG_CODEC2RAW_MUXER)           += rawenc.o
 OBJS-$(CONFIG_CONCAT_DEMUXER)            += concatdec.o
 OBJS-$(CONFIG_CRC_MUXER)                 += crcenc.o
 OBJS-$(CONFIG_CUFRAMES_DEMUXER)          += cuframesdec.o
+OBJS-$(CONFIG_CUFRAMES_PACKETS_DEMUXER)  += cuframes_packetsdec.o
 OBJS-$(CONFIG_DATA_DEMUXER)              += rawdec.o
 OBJS-$(CONFIG_DATA_MUXER)                += rawenc.o
 OBJS-$(CONFIG_DASH_MUXER)                += dash.o dashenc.o hlsplaylist.o