feat(network): implement Phase 4 RTP framing with FU-A fragmentation

Add the sc_network library: RFC 3550 RtpHeader/RtpPacket serialize and
parse (the receiver tolerates CSRC lists, extension headers, and
padding by skipping/stripping them) and RFC 6184 H.264 payloading via
H264Packetizer/H264Depacketizer.

The packetizer splits Annex-B frames into NAL units (3- and 4-byte
start codes), emitting single-NAL packets or FU-A fragments within the
configured MTU, with the marker bit closing each frame and randomized
SSRC/sequence by default. The depacketizer reassembles access units
with 3-byte start codes, so both start-code widths round-trip
byte-exactly; frames damaged by sequence gaps or missing fragments
are dropped until the Phase 7 loss-recovery work.

test_rtp covers header and packet round-trips, malformed-input
rejections, splitter behavior, FU-A chunk bounds, full packetize ->
depacketize round-trip, gap dropping, marker-only frame separation,
sequence wrap, and empty inputs. meson test 3/3, valgrind clean.

tests/meson.build

@@ -9,3 +9,9 @@ test_codec_roundtrip = executable('test_codec_roundtrip',
     dependencies : sc_codec_dep)
 
 test('h264 roundtrip', test_codec_roundtrip)
+
+test_rtp = executable('test_rtp',
+    'network/test_rtp.cpp',
+    dependencies : sc_network_dep)
+
+test('rtp framing', test_rtp)