docs(apple): pickup-ready stage-2 presenter implementation plan

Stage-2 was a one-line "next" in the README. Add a full, actionable spec
(docs-site apple-stage2-presenter.md) a Mac agent can execute: VTDecompressionSession
decode (with decode-completion stamping) -> CAMetalLayer + display-link present, the
exact integration points against the existing StreamPump/StreamView/AnnexB/LatencyMeter,
the three-stage measurement wiring (capture->decoded / decode->present / capture->present
= glass-to-glass, using the already-wired PunktfunkConnection.clockOffsetNs), a cheaper
decode-only intermediate, validation, and gotchas. Link it from the Apple README's
Stage 2 item. (meta.json nav entry left in the working tree to land with the CI docs WIP.)

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

clients/apple/README.md

@@ -167,9 +167,12 @@ signing, bundle id `io.unom.punktfunk`. Notes:
    and recovery keyframes re-send them — "refresh the format description on every IDR"
    (what `StreamView` does) is sufficient; there is no out-of-band extradata, ever.
 4. **Stage 2 (next)**: explicit `VTDecompressionSession` + `CAMetalLayer` for frame-pacing
-   control (ProMotion/120 Hz), glass-to-glass measurement via `tools/latency-probe` (the
+   control (ProMotion/120 Hz) and true decode→present / glass-to-glass measurement. The
-   host stamps `pts_ns` with its capture wall clock; across machines you need a clock
+   cross-machine clock offset is **already wired** — `PunktfunkConnection.clockOffsetNs` (from
-   offset estimate from the QUIC RTT).
+   the connect-time skew handshake); add it to a `CLOCK_REALTIME` present instant and subtract
+   the AU `pts_ns`. **Full pickup-ready implementation plan** (decode + present + measurement
+   wiring, integration points, gotchas): `docs-site/content/docs/apple-stage2-presenter.md`
+   (rendered in the docs site under "Apple Stage-2 Presenter").
 5. **Audio — wired, both directions.** Playback: `SessionAudio` drains `nextAudio()`
    on its own thread, decodes through CoreAudio's built-in Opus codec (`OpusCodec.swift`
    — kAudioFormatOpus, no bundled libopus; round-trip unit-tested) into a priming

docs-site/content/docs/apple-stage2-presenter.md

@@ -0,0 +1,122 @@
+---
+title: "Apple Stage-2 Presenter (handoff)"
+description: "Implementation plan for the explicit VTDecompressionSession → CAMetalLayer presenter — hand-paced present + true decode→present (glass-to-glass) measurement. Written so a Mac agent can pick it up."
+---
+
+A pickup-ready plan for the **stage-2 Apple presenter**. The current **stage-1** presenter feeds
+compressed HEVC straight into `AVSampleBufferDisplayLayer`, which hardware-decodes **and presents
+internally with no per-frame callback** — so we can't stamp decode or present, and we can't hand-pace.
+Stage-2 takes explicit control: decode with `VTDecompressionSession`, present decoded frames through a
+`CAMetalLayer` driven by a display link. Two wins: **~0.5 refresh off the present tail** (the biggest
+client latency term at 60 Hz) and **true decode→present / glass-to-glass** numbers.
+
+All of this is **macOS/iOS/tvOS-only** — build + validate on a Mac (`swift build && swift test`, then
+live against a Linux host). The host + connector side is already done: `PunktfunkConnection.clockOffsetNs`
+(the connect-time skew offset, host minus client) is what makes the present timestamp cross-machine
+valid. See [Status](/docs/status) and roadmap §12.
+
+## Where it plugs into the existing code
+
+| Existing (stage-1) | Stage-2 change |
+|---|---|
+| `StreamPump` pulls AUs → `AnnexB.sampleBuffer` → `layer.enqueue` (compressed) | A `Stage2Pump` (or a mode flag on `StreamPump`) feeds AUs to `VTDecompressionSessionDecodeFrame` instead |
+| `StreamView`/`StreamViewIOS` host an `AVSampleBufferDisplayLayer` | Host a `CAMetalLayer` (+ a display link); keep the input-capture + HUD overlay unchanged |
+| `AnnexB.formatDescription(fromIDR:)` builds the format desc, refreshed on every IDR | **Reused** — it's the `VTDecompressionSession`'s format description; recreate the session when it changes |
+| `LatencyMeter` records capture→client-receipt at `onFrame` | Extend to record **decode-completion** and **present** stages (below) |
+
+Keep stage-1 behind a `UserDefaults` flag (e.g. `punktfunk.presenter = "stage1" | "stage2"`) so a
+regression can fall back — `AVSampleBufferDisplayLayer` is the known-good path.
+
+## Decode: VTDecompressionSession
+
+1. Create the session from the IDR's `CMVideoFormatDescription`
+   (`AnnexB.formatDescription(fromIDR:)`):
+   ```
+   VTDecompressionSessionCreate(
+     allocator: nil,
+     formatDescription: fmt,
+     decoderSpecification: nil,           // hardware by default; no need to force
+     imageBufferAttributes: [
+       kCVPixelBufferMetalCompatibilityKey: true,
+       kCVPixelBufferPixelFormatTypeKey:
+         kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange, // 8-bit SDR; 10-bit (…10BiPlanar) for HDR later
+     ],
+     outputCallback: <C-callback>,
+     decompressionSessionOut: &session)
+   ```
+2. Per AU: build the same `CMSampleBuffer` as stage-1 (`AnnexB.sampleBuffer(au:format:)`, PTS =
+   `au.ptsNs` @ 1e9 timescale) and submit:
+   ```
+   VTDecompressionSessionDecodeFrame(session, sampleBuffer,
+     flags: ._EnableAsynchronousDecompression,
+     frameRefcon: <pts or a boxed context>, infoFlagsOut: nil)
+   ```
+3. The **output callback** delivers `(status, infoFlags, imageBuffer: CVImageBuffer?, presentationTimeStamp, …)`.
+   `presentationTimeStamp` is `au.ptsNs` (the host capture clock). **Stamp decode-completion here**
+   (`CLOCK_REALTIME` ns), retain the `CVPixelBuffer`, and push `{pts, pixelBuffer, decodedNs}` into a
+   small NSLock-guarded ring (the "ready" queue) the display link drains.
+4. **IDR / mode change**: when `AnnexB.formatDescription` yields a new desc, check
+   `VTDecompressionSessionCanAcceptFormatDescription`; if not, finish-and-recreate the session (same
+   trigger stage-1 uses to refresh `format`). On decoder error (`kVTVideoDecoderBadDataErr`, etc.) drop
+   to the next IDR — there's no out-of-band extradata; recovery keyframes re-carry the parameter sets.
+
+## Present: CAMetalLayer + display link
+
+- `CAMetalLayer` (device = system default, `pixelFormat = .bgra8Unorm`, `framebufferOnly = true`,
+  `drawableSize` = stream WxH). The view: macOS `NSView`/iOS `UIView` whose `layerClass`/backing layer
+  is the `CAMetalLayer` (mirror `StreamView`/`StreamViewIOS`).
+- **Display link** drives present: macOS `CVDisplayLink` (or `CADisplayLink` on macOS 14+),
+  iOS/tvOS `CADisplayLink`. Each callback carries the **target present timestamp** (`CVTimeStamp` /
+  `targetTimestamp`).
+- Each vsync: pop the **newest** ready frame (drop older undisplayed ones — low-latency default; no
+  smoothing buffer to start), render a fullscreen quad sampling the **biplanar YUV** (luma +
+  chroma planes via `CVMetalTextureCache`) with a BT.709 YUV→RGB fragment shader, then
+  `commandBuffer.present(drawable)` (or `present(drawable, atTime:)`). **Stamp present time** for the
+  frame just shown (use the display link's target timestamp converted to `CLOCK_REALTIME`).
+- Colorspace: BT.709 8-bit for now (matches the host's SDR). HDR (BT.2020/PQ, 10-bit `…10BiPlanar` +
+  EDR `CAMetalLayer.wantsExtendedDynamicRangeContent`) is a later tie-in with the HDR roadmap (§10).
+
+### Cheaper intermediate (2a) if the Metal path is too big in one step
+Decode with `VTDecompressionSession` (gets the **decode-completion timestamp** = capture→decoded),
+then wrap the decoded `CVPixelBuffer` in a `CMSampleBuffer` and `enqueue` it into the existing
+`AVSampleBufferDisplayLayer` (it accepts uncompressed pixel buffers too). This yields the decode term
+**without** a Metal renderer — but **not** true present (the layer still presents internally). Ship 2a
+first if useful; 2b (CAMetalLayer + display link) is required for the on-glass present stamp.
+
+## Measurement (the whole point)
+
+Extend `LatencyMeter` (or add per-stage meters) so each frame records three instants, all
+`CLOCK_REALTIME` ns, all shifted by `connection.clockOffsetNs` to the host clock:
+
+- **capture→decoded** = `decodedNs + offset − pts_ns` (VideoToolbox decode latency, cross-machine)
+- **decode→present** = `presentedNs − decodedNs` (the present tail stage-2 shortens)
+- **capture→present** = `presentedNs + offset − pts_ns` — **the glass-to-glass number** (modulo the
+  host render→capture term, still unmeasured; see roadmap §12)
+
+Surface `capture→present` p50/p95 in the HUD (extend the existing `model.latency*` line in
+`ContentView`). `skewCorrected` stays false when `clockOffsetNs == 0` (old host) — then the numbers are
+same-host-only, as today.
+
+## Validation
+
+- `swift test`: add a decode-output test (decode a known IDR built like
+  `VideoToolboxRoundTripTests` → assert a `CVPixelBuffer` of the right dimensions + the
+  decode callback fires). Present is display-bound — validate it **live** via the HUD number.
+- Live: connect to a Linux host (`m3-host --source virtual` on the GNOME box; see
+  [GNOME Box Setup](/docs/gnome-box)), confirm `capture→present` is a few ms over `capture→client`
+  and that `decode→present` shrank vs. an `AVSampleBufferDisplayLayer` baseline.
+- Compare against the headless reference number: `punktfunk-client-rs` reports skew-corrected
+  capture→reassembled (~1.3 ms p50 GNOME box → dev box); capture→present should be that **+ decode +
+  present**.
+
+## Gotchas
+
+- VT decode is **async**; the output callback runs on a VT-managed thread — don't block it, just stamp
+  + enqueue. Retain the `CVPixelBuffer` until presented (the ring owns it).
+- `VTDecompressionSessionDecodeFrame` wants the **same** `CMSampleBuffer` shape stage-1 builds (AVCC
+  length-prefixed NALs, in-band parameter sets in the format desc, never as extradata).
+- `CAMetalLayer.drawableSize` must track mode changes (the host can `Reconfigure` mid-stream — watch
+  `PunktfunkConnection.mode`/the new-IDR dimensions).
+- Don't add a jitter/smoothing buffer for the first cut — present newest-ready for lowest latency; a
+  pacing policy can come later if frames look uneven.
+- Keep `clients/apple/README.md`'s "Stage 2" item + [Status](/docs/status) updated when this lands.