feat(clients): host/network split in every stats HUD (stats phase 2, client side)

Consumes the 0xCF host-timing plane (449a67c) on all four GUI clients: each
keeps a bounded pending ring of receipt samples keyed by pts, matches the
host's per-AU capture→sent reports against it, and the HUD equation becomes

  = host 3.1 + network 6.7 + decode 2.1 + display 2.3

falling back to the combined `= host+network …` term whenever no timing
matched the window (old host / datagram loss) — same total, one split
fewer, never a misleading zero. Apple additionally gains the split as the
only equation line under the stage-1 fallback presenter (receipt is
presenter-independent), a `nextHostTiming` wrapper with its own plane lock,
and a unit-tested `HostNetworkSplitter`; Android extends the JNI stats
array 16→18 doubles (0–15 unchanged); Windows/Linux thread the split
through `Stats` into the HUD and the headless/debug logs.

Docs updated: design/stats-unification.md Phase 2 → implemented (wire
format, fallback semantics), and the docs-site matrix's Sunshine "Host
processing latency" row is now a direct match (ours includes the paced
send; avg vs p50).

Verified here: linux client clippy -D warnings green on the live tree,
windows stub check + hand-verified diff, android cargo-ndk arm64 check
green, apple loopback test extended (needs the rebuilt xcframework + swift
test on the mac). On-glass: pending on all platforms.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/ContentView.swift

@@ -326,9 +326,14 @@ struct ContentView: View {
                     onCaptureChange: { [weak model] captured in
                         model?.mouseCaptured = captured
                     },
-                    onFrame: { [meter = model.meter, latency = model.latency, offset = conn.clockOffsetNs] au in
+                    onFrame: { [meter = model.meter, latency = model.latency,
+                                split = model.latencySplit, offset = conn.clockOffsetNs] au in
                         meter.note(byteCount: au.data.count)
                         latency.record(ptsNs: au.ptsNs, offsetNs: offset)
+                        // The same receipt, keyed by pts, awaiting its 0xCF host timing (the
+                        // host/network split — drained by the 1 s stats tick).
+                        split.recordReceipt(
+                            ptsNs: au.ptsNs, receivedNs: au.receivedNs, offsetNs: offset)
                     },
                     onSessionEnd: { [weak model] in
                         Task { @MainActor in model?.sessionEnded() }