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.

.agents/MEMORY.md

@@ -1,13 +1,27 @@
 # Project Memory — screen_cast
 
-Last updated: Phase 3 validated and complete; current phase is Phase 4.
+Last updated: Phase 4 (RTP framing) complete and tested; current phase is
+Phase 5.
 
 ## Project state
 
-- **Phase 3 is done and validated on the desktop**: a manual
-  `./build/tools/capture_smoke 10 out.h264` run on Wayland/Hyprland produced
-  a valid 2256x1504 H.264 elementary stream (ffprobe clean). `docs/PHASES.md`
-  is ticked; current phase is Phase 4 — RTP framing.
+- Phase 4 done: `sc_network` library implements `RtpHeader`/`RtpPacket`
+  (RFC 3550 serialize/parse; tolerates CSRC lists, extension headers, and
+  padding on the receive side) plus `H264Packetizer`/`H264Depacketizer`
+  (RFC 6184 single-NAL + FU-A; STAP-A never emitted, unsupported types mark
+  the frame damaged on receive).
+- Depacketized access units use **3-byte start codes**, and the splitter
+  keeps a zero byte preceding a start code with the previous NAL, so both
+  3- and 4-byte-start-code streams round-trip byte-exactly (tested).
+- Loss handling is drop-on-damage: a sequence gap or missing FU fragment
+  marks the frame damaged and it is dropped silently at its marker. Full
+  loss recovery / jitter handling is Phase 7.
+- `test_rtp` covers header/packet round-trips, malformed rejections, NAL
+  splitting (3- and 4-byte codes), FU-A chunking with MTU bounds, full
+  packetize→depacketize round-trip, gap dropping, frame separation by
+  marker alone, sequence wrap, and random default SSRC/sequence.
+  Valgrind-clean; `meson test` 3/3.
+- Phase 3 remains validated; earlier review fixes still in place.
 - The first smoke run emitted `impl_ext_end_proxy called from wrong context`
   warnings: `pw_context_connect_fd` and `pw_core_disconnect` ran outside the
   thread-loop lock. Fixed by holding the lock across all pw setup/teardown