fix(core): grow UDP socket buffers — fixes 4K/5K video freezing on one frame

The data-plane UDP sockets used the OS default buffer (~208 KB on Linux, similar
on macOS), which is smaller than a single high-resolution frame burst: a
5120×1440 keyframe is ~130 packets the encode|send thread hands to sendmmsg at
once. The burst overflows the buffer — EAGAIN on the host send (now dropped, was
fatal) or a silent drop on the client recv — and because the data plane runs
infinite-GOP, one lost frame breaks every subsequent reference and the decode
freezes on the last good frame until an RFI refresh that may never catch up.
Symptom: connect at 5120×1440, see ONE frame, then a frozen image (audio + input
keep working — those ride QUIC, not this socket).

Set SO_SNDBUF/SO_RCVBUF to 8 MB (clamped by the OS to net.core.{w,r}mem_max on
Linux / kern.ipc.maxsockbuf on macOS); warn if the grant lands far below target so
an undersized host is diagnosable. The client side matters most — the SAME
UdpTransport backs the Apple client's data plane via the C ABI, and macOS grants
multi-MB buffers without any sysctl, so a rebuilt client stops losing frames.

Validated live, bazzite→client at 5120×1440: was 1319/1500 frames (12% loss →
freeze), now 1500/1500 @60 and 5279/5279 @240 (split-encode active), zero
mismatches, p50 1.9–3.4 ms. Host send buffer was still capped at 416 KB and lost
nothing — the loss was purely the client recv buffer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -1978,6 +1978,7 @@ dependencies = [
  "rustls",
  "rustls-pki-types",
  "sha2",
+ "socket2",
  "spake2",
  "thiserror 2.0.18",
  "tokio",