fix(apple): default iOS to glass-gated present pacing with a depth-2 gate — the 23 ms display-stage fix

Field report (iPad Pro 13" M4, 2752x2064@120): display 23.1 ms on the default
arrival pacing vs 14 ms glass-gated, dominating an otherwise ~14 ms pipeline.
On iOS the layer ALWAYS vsync-latches (displaySyncEnabled is macOS-only API)
and default-on VRR steers the panel to the stream rate, so arrival pacing's
sticky-FIFO saturation (~2-3 refreshes of queue) is the common case, not the
corner — the exact regime that made tvOS default to glass.

- PresenterChoice.platformDefault -> stage3 on iOS/iPadOS (joins tvOS);
  explicit stage-2 stays the honest arrival A/B; macOS unchanged.
- PresentGate generalized to a capacity: depth 1 is bit-identical to before;
  iOS runs depth 2 (one flip scanning out + one queued for the next latch),
  so a decoded frame presents immediately and latches the very next vsync
  instead of serializing on the previous flip's on-glass callback — expected
  ~5-8 ms at 120 Hz. tvOS keeps depth 1 (proven; A/B first), macOS is pinned
  to 1 (glass there is the DCP swapID-panic mitigation — serialization is
  its point). PUNKTFUNK_GATE_DEPTH (1-3) is the on-device A/B lever.
- Settings picker derives its "(default)" marker from presenterDefault
  instead of hardcoding stage-2 as default / stage-3 as experimental.

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

clients/apple/Sources/PunktfunkClient/Settings/SettingsOptions.swift

@@ -50,14 +50,14 @@ enum SettingsOptions {
         return options
     }
 
-    /// The platform's presenter default (mirrors SessionPresenter's platformDefault — tvOS runs
-    /// glass pacing, everything else arrival). Views seed their @AppStorage display from this so
-    /// an untouched picker shows what actually runs.
+    /// The platform's presenter default (mirrors SessionPresenter's platformDefault — tvOS and
+    /// iOS/iPadOS run glass pacing, macOS arrival). Views seed their @AppStorage display from
+    /// this so an untouched picker shows what actually runs.
     static var presenterDefault: String {
-        #if os(tvOS)
-        "stage3"
-        #else
+        #if os(macOS)
         "stage2"
+        #else
+        "stage3"
         #endif
     }
 

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

@@ -412,27 +412,28 @@ extension SettingsView {
         #endif
     }
 
-    // Stage-2 (Metal/VTDecompressionSession, present on frame arrival) is the proven default;
-    // stage-3 is the same pipeline with glass-gated present pacing — a user-visible A/B while the
-    // pacing work settles (see Stage2Pipeline's PresentPacing for the queue-saturation rationale).
-    // Stage-1 (compressed video straight to the system layer) stays a DEBUG-only diagnostic — it
-    // freezes hard on a lost HEVC reference.
+    // Stage-2 (Metal/VTDecompressionSession, present on frame arrival) vs stage-3 (the same
+    // pipeline with glass-gated present pacing). The default is per-platform — glass on iOS/tvOS,
+    // whose layers always vsync-latch, arrival on macOS (see Stage2Pipeline's PresentPacing for
+    // the queue-saturation rationale) — so the "(default)" marker rides presenterDefault instead
+    // of a hardcoded row. Stage-1 (compressed video straight to the system layer) stays a
+    // DEBUG-only diagnostic — it freezes hard on a lost HEVC reference.
     @ViewBuilder var presenterSection: some View {
         Section {
             Picker("Presenter", selection: $presenter) {
-                Text("Stage 2 (default)").tag("stage2")
-                Text("Stage 3 (experimental)").tag("stage3")
-                #if DEBUG
-                Text("Stage 1 (debug)").tag("stage1")
-                #endif
+                ForEach(SettingsOptions.presenters, id: \.tag) { option in
+                    Text(option.tag == SettingsOptions.presenterDefault
+                        ? "\(option.label) (default)" : option.label)
+                        .tag(option.tag)
+                }
             }
         } header: {
             Text("Video presenter")
         } footer: {
-            Text("Stage 2: each frame is shown the moment it's decoded — proven, but on displays "
+            Text("Stage 2: each frame is shown the moment it's decoded — but on displays "
                 + "running near the stream's frame rate, queued frames can add two to three "
                 + "refreshes of display latency that never drains. Stage 3: presents are paced "
-                + "to the display — at most one undisplayed frame in flight, always the freshest, "
+                + "to the display — a strict cap on undisplayed frames, always the freshest, "
                 + "dropping late frames instead of queueing them. Watch the statistics overlay's "
                 + "display time to compare. Applies from the next session.")
                 .font(.geist(12, relativeTo: .caption))