feat(net/mac): default-on recvmsg_x batched Mac recv + GSO host + longer probe

The Mac/iOS client's wall around ~380 Mbps on a 2.5 G path is the receive
drain, not the transport: a loopback speed-test pushes 380/600/1000 Mbps at
0.0% loss, but Darwin has no recvmmsg(2), so the macOS client was doing one
recv() syscall per packet — ~40-90k syscalls/s on one core. When the recv loop
can't drain fast enough the kernel socket buffer backs up and drops, which the
client sees as a sustained stream stalling/freezing in the 300-400 Mbps range
(and an immediate "session ended" when a 500 Mbps+ first keyframe bursts in).

- core/transport: flip recvmsg_x (the batched Darwin recv, ~30x fewer syscalls)
  from opt-in to default ON, opt-out via PUNKTFUNK_RECVMSG_X=0. Keeps the
  auto-fallback to the scalar loop on any unexpected syscall error. The Apple CI
  swift-test loopback now exercises this path by default.
- packaging/kde host.env: enable PUNKTFUNK_GSO=1 — UDP segmentation offload on
  the host send path (one sendmsg per ~64 packets), the dominant lever above
  ~1 Gbps. Already wired (send_sealed -> send_gso) with sendmmsg auto-fallback.
- apple SpeedTestSheet: lengthen the bandwidth probe 2 s -> 5 s so the measured
  number stops swinging wildly (50 vs 900 Mbps on the same link) — long enough
  for steady-state send + recv drain to settle. Matches host MAX_PROBE_MS.
- host capture: PUNKTFUNK_SYNTH_NOISE synthetic high-entropy source for
  reproducible throughput testing of the encode->FEC->send->recv path.

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

clients/apple/Sources/PunktfunkClient/SpeedTestSheet.swift

@@ -23,10 +23,12 @@ private final class ProbeToken: @unchecked Sendable {
 /// What the host is asked to burst: the host's full probe ceiling (it clamps to ≤ 3 Gbps),
 /// so the measurement surfaces the link's real ceiling instead of an artificial cap —
 /// bursting ABOVE what the link can carry is how the probe finds where delivery falls off.
-/// Two seconds rides out scheduler jitter. File-scope so the detached probe task reads them
-/// without crossing into the view's main actor.
+/// Five seconds (was 2 s) averages out the scheduler/recv jitter that made a short probe swing
+/// wildly (50 vs 900 Mbps on the same link) — long enough for the host's steady-state send and
+/// the client's recv drain to settle. File-scope so the detached probe task reads them without
+/// crossing into the view's main actor.
 private let probeTargetKbps: UInt32 = 3_000_000
-private let probeDurationMs: UInt32 = 2_000
+private let probeDurationMs: UInt32 = 5_000
 
 struct SpeedTestSheet: View {
     @Environment(\.dismiss) private var dismiss