feat(apple): stage-2 presenter — explicit decode + Metal present + glass-to-glass

Opt-in (Settings -> Presenter; `punktfunk.presenter`, default stage-1). Stage-1's
AVSampleBufferDisplayLayer decodes AND presents internally with no per-frame
callback, so neither decode nor present can be stamped or hand-paced. Stage-2
takes explicit control:

- VideoDecoder: VTDecompressionSession, async output callback stamps
  decode-completion, session rebuilt on every IDR / format change. Unit-tested
  (testVideoDecoderAsyncCallbackDeliversPixels).
- MetalVideoPresenter: CAMetalLayer + CVMetalTextureCache + a runtime-compiled
  BT.709 limited-range NV12->RGB shader, present at the next vsync. The
  CVMetalTextures + pixel buffer are held until the GPU completes.
- Stage2Pipeline: pump thread -> decoder -> newest-ready 1-slot ring; the hosting
  view's display link drains it once per vsync and stamps capture->present
  (the display-link target time projected into CLOCK_REALTIME).
- LatencyMeter gains record(ptsNs:atNs:offsetNs:); the HUD shows a capture->present
  (glass-to-glass, modulo host render->capture) line, skew-corrected via
  clockOffsetNs. Measured live ~11 ms p50 vs ~2.2 ms capture->client.
- StreamView / StreamViewIOS host the CAMetalLayer as a sublayer + a CADisplayLink
  (NSView.displayLink on macOS) when stage-2; input capture + HUD unchanged. The
  session-active gates switch from `pump != nil` to `connection != nil` so capture
  engages without a StreamPump.

Validated: builds macOS/iOS/tvOS; the decode half is unit-tested; the Metal
present is live-validated on glass (correct image + the capture->present number).
Colorspace is BT.709 SDR for now; 10-bit/HDR + a pacing policy are later.
Plan: docs-site/content/docs/apple-stage2-presenter.md.

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

clients/apple/Sources/PunktfunkClient/SettingsView.swift

@@ -17,6 +17,7 @@ struct SettingsView: View {
     @AppStorage("punktfunk.compositor") private var compositor = 0
     @AppStorage("punktfunk.gamepadType") private var gamepadType = 0
     @AppStorage("punktfunk.bitrateKbps") private var bitrateKbps = 0
+    @AppStorage("punktfunk.presenter") private var presenter = "stage1"
     @AppStorage("punktfunk.micEnabled") private var micEnabled = true
     @ObservedObject private var gamepads = GamepadManager.shared
     #if os(macOS)
@@ -88,6 +89,10 @@ struct SettingsView: View {
                 }
                 TVSelectionRow(
                     title: "Compositor", options: compositors, selection: $compositor)
+                TVSelectionRow(
+                    title: "Presenter",
+                    options: [("Stage 1 (default)", "stage1"), ("Stage 2 (experimental)", "stage2")],
+                    selection: $presenter)
                 Text("The host creates a virtual output at exactly this mode — native "
                     + "resolution, no scaling. \(Self.bitrateFooter) A specific compositor "
                     + "is honored only if available on the host.")
@@ -366,6 +371,21 @@ struct SettingsView: View {
                     .font(.caption)
                     .foregroundStyle(.secondary)
             }
+            Section {
+                Picker("Presenter", selection: $presenter) {
+                    Text("Stage 1 (default)").tag("stage1")
+                    Text("Stage 2 (experimental)").tag("stage2")
+                }
+            } header: {
+                Text("Video presenter")
+            } footer: {
+                Text("Stage 1 feeds compressed video to the system display layer (known-good). "
+                    + "Stage 2 decodes explicitly and presents through Metal with a display "
+                    + "link — it adds a capture→present (glass-to-glass) latency line in the HUD "
+                    + "and shortens the present tail. Applies from the next session.")
+                    .font(.caption)
+                    .foregroundStyle(.secondary)
+            }
             Section {
                 if gamepads.controllers.isEmpty {
                     Text("No controllers detected")