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/VideoToolboxRoundTripTests.swift

@@ -38,7 +38,8 @@ final class VideoToolboxRoundTripTests: XCTestCase {
         XCTAssertEqual(AnnexB.avcc(from: annexB, codec: .hevc), avccSample)
 
         // 3) Sample buffer → real decoder → pixels.
-        let au = AccessUnit(data: annexB, ptsNs: 1_000_000, frameIndex: 0, flags: 0)
+        let au = AccessUnit(
+            data: annexB, ptsNs: 1_000_000, frameIndex: 0, flags: 0, receivedNs: 0)
         let sample = try XCTUnwrap(AnnexB.sampleBuffer(au: au, format: rebuilt, codec: .hevc))
 
         var session: VTDecompressionSession?
@@ -67,13 +68,14 @@ final class VideoToolboxRoundTripTests: XCTestCase {
     }
 
     /// Stage-2 decode half: the same known IDR through `VideoDecoder` — assert its async output
-    /// callback fires with a CVPixelBuffer of the right dimensions, the pts round-trips, and
-    /// decode-completion is stamped.
+    /// callback fires with a CVPixelBuffer of the right dimensions, the pts and the receipt stamp
+    /// round-trip (the latter rides the frame refcon), and decode-completion is stamped.
     func testVideoDecoderAsyncCallbackDeliversPixels() throws {
         let (formatDesc, avccSample) = try encodeOneHEVCKeyframe()
         let annexB = try annexBAU(formatDesc: formatDesc, avccSample: avccSample)
         let format = try XCTUnwrap(AnnexB.formatDescription(fromIDR: annexB, codec: .hevc))
-        let au = AccessUnit(data: annexB, ptsNs: 42_000_000, frameIndex: 0, flags: 0)
+        let au = AccessUnit(
+            data: annexB, ptsNs: 42_000_000, frameIndex: 0, flags: 0, receivedNs: 41_000_000)
 
         let box = FrameBox()
         let done = DispatchSemaphore(value: 0)
@@ -100,6 +102,8 @@ final class VideoToolboxRoundTripTests: XCTestCase {
         XCTAssertEqual(CVPixelBufferGetWidth(ready.pixelBuffer), width)
         XCTAssertEqual(CVPixelBufferGetHeight(ready.pixelBuffer), height)
         XCTAssertEqual(ready.ptsNs, 42_000_000, "pts round-trips through the decoder")
+        XCTAssertEqual(
+            ready.receivedNs, 41_000_000, "receivedNs round-trips through the frame refcon")
         XCTAssertGreaterThan(ready.decodedNs, 0, "decode-completion is stamped")
     }