feat(clients): unified stats vocabulary across every client + Moonlight comparison docs

One stat model everywhere (design/stats-unification.md): four measurement
points (capture/received/decoded/displayed), three stages that tile the
interval exactly, and a HUD that shows the addition explicitly —

  end-to-end 14.2 ms p50 · 19.8 p95 · capture→on-glass
  = host+network 9.8 + decode 2.1 + display 2.3

replacing each client's ad-hoc mix of overlapping absolutes (the Apple HUD's
three arrow lines that looked sequential but weren't), mean-vs-median decode
times (Windows/Linux), missing same-host-clock flags (Windows/Linux), and
three different names for the same capture→received measurement (probe's
"reassembled", Apple/Android's "client", Windows/Linux's post-decode "lat").

Per client: Apple threads receivedNs through the VT decode via the frame
refcon bit pattern so the decode stage exists at all (stage-1 fallback
honestly degrades to a capture→received headline); Windows carries
FrameTimes through the existing frame channel to the render thread and adds
e2e p50/p95 post-Present; Linux stamps received at AU pop and rides
decoded_ns on DecodedFrame to the paintable-set site; Android pairs receipt
stamps with MediaCodec output buffers via the codec's pts round-trip (JNI
stats array 14→16 doubles, indexes 0-13 unchanged). fps now uniformly counts
received AUs; lost/(received+lost) per window, hidden at zero.

docs-site gains "Understanding the Stats Overlay": what each line means, why
the equation only approximately sums (percentiles), and a line-by-line
Moonlight/Sunshine matrix — including that Moonlight has no end-to-end
number and its "network latency" is an ENet control RTT, so punktfunk's
headline must not be compared against any single Moonlight line.

Verified here: linux client + probe + core check/clippy/fmt green, android
native cargo-ndk arm64 check green. Pending: Windows CI + on-glass, swift
test on the mac, on-device Android.

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

clients/apple/Sources/PunktfunkClient/Settings/SettingsView+Sections.swift

@@ -310,10 +310,11 @@ extension SettingsView {
             Text("Video presenter · debug")
         } footer: {
             Text("Stage 2 (default) decodes explicitly and presents through Metal with a display "
-                + "link — it adds a capture→present (glass-to-glass) latency line in the HUD and "
-                + "self-recovers from decode stalls. Stage 1 feeds compressed video straight to the "
-                + "system display layer; it freezes on a lost HEVC reference frame, so it's a debug "
-                + "fallback only. Applies from the next session.")
+                + "link — it gives the HUD the end-to-end (capture→on-glass) headline with the "
+                + "host+network/decode/display stage equation and self-recovers from decode "
+                + "stalls. Stage 1 feeds compressed video straight to the system display layer; "
+                + "it freezes on a lost HEVC reference frame, so it's a debug fallback only. "
+                + "Applies from the next session.")
                 .font(.geist(12, relativeTo: .caption))
                 .foregroundStyle(.secondary)
         }