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/Tests/PunktfunkKitTests/LatencyMeterTests.swift

@@ -1,6 +1,10 @@
-// Unit tests for LatencyMeter: percentiles, the skew-corrected flag, reset-on-drain, and the
-// absurd-value guard. Latencies are constructed by stamping a pts a known interval in the past, so
-// the result is that interval plus the (tiny) clock advance between reads — asserted with tolerance.
+// Unit tests for LatencyMeter (one instance per unified-stats stage — see
+// design/stats-unification.md): percentiles, the skew-corrected flag, reset-on-drain, the
+// absurd-value guard, and the explicit-instant stage form (record(ptsNs:atNs:offsetNs:), used for
+// the client-local decode/display stages and the at-present end-to-end stamp). Receipt-path
+// latencies are constructed by stamping a pts a known interval in the past, so the result is that
+// interval plus the (tiny) clock advance between reads — asserted with tolerance; the explicit
+// form is exact.
 
 import Foundation
 import XCTest
@@ -38,6 +42,26 @@ final class LatencyMeterTests: XCTestCase {
         XCTAssertEqual(m.drain()?.skewCorrected, true)
     }
 
+    func testExplicitStageRecordIsExact() {
+        let m = LatencyMeter()
+        // A client-local stage (decode: received→decoded) — start instant as ptsNs, offset 0.
+        let receivedNs: Int64 = 1_000_000_000_000
+        m.record(ptsNs: UInt64(receivedNs), atNs: receivedNs + 3_000_000, offsetNs: 0)
+        guard let s = m.drain() else { return XCTFail("expected a sample") }
+        XCTAssertEqual(s.count, 1)
+        XCTAssertEqual(s.p50Ms, 3.0, "explicit instants make the sample exact")
+        XCTAssertFalse(s.skewCorrected, "local stages record with offset 0")
+    }
+
+    func testExplicitStageDropsNonPositiveInterval() {
+        let m = LatencyMeter()
+        // A stage whose start stamp is missing (0) or after its end must not pollute the window.
+        let decodedNs: Int64 = 1_000_000_000_000
+        m.record(ptsNs: 0, atNs: decodedNs, offsetNs: 0) // "start unknown" → > 10 s → dropped
+        m.record(ptsNs: UInt64(decodedNs + 1), atNs: decodedNs, offsetNs: 0) // negative → dropped
+        XCTAssertNil(m.drain())
+    }
+
     func testDropsAbsurdValues() {
         let m = LatencyMeter()
         let now = nowRealtimeNs()