fix(apple): stage-4 deadline presenter (CAMetalDisplayLink) — iOS default; gate depth back to 1

Field verdict on the depth-2 gate (M4 iPad Pro, 2752x2064@120): display stage
22-28 ms vs depth-1's 14 — a REGRESSION, not the predicted 5-8. Post-mortem:
any bounded-FIFO pacing keeps a STANDING queue on the always-vsync-latch
platforms. One burst fills every admitted slot and, with arrivals and latches
then running at the same rate, occupancy never returns to zero — each gate
slot costs one full refresh, permanently (the ladder fits exactly: arrival ~3
slots -> 30+ ms, depth 2 -> 22-28, depth 1 -> 14). A bounded FIFO caps the
queue; nothing ever drains it. And depth 1 serializes presents on the on-glass
callback's delivery lag, so neither rung can approach the sub-refresh floor.

Stage-4 (PresentPacing.deadline) inverts drawable ownership instead:
- A CAMetalDisplayLink on its own runloop thread vends ONE deadline-timed
  drawable per refresh (preferredFrameLatency 1) into a newest-wins LatestBox;
  an unpresented vend is replaced by the next (back to the pool).
- The render thread pairs it with the newest decoded frame the moment either
  half arrives — the common case presents a frame INSTANTLY into an already-
  vended drawable, latching the upcoming refresh. No image queue can form and
  nothing waits on on-glass callbacks (they only feed the meters now).
- A stashed drawable can lag a mid-session HDR reconfigure by one vend:
  encodePresent skips the mismatched-format vend (frame re-rings) instead of
  tripping Metal validation.
- iOS/iPadOS defaults to stage-4; tvOS keeps stage-3 until its own A/B
  (PUNKTFUNK_PRESENTER=stage4); macOS resolves "stage4" back to its default
  (the sync-off/DCP-panic saga — deadline pacing lands there deliberately or
  not at all). Settings picker gains Stage 4 with the derived default marker.
- Gate depth defaults to 1 everywhere again; PUNKTFUNK_GATE_DEPTH stays only
  to reproduce the standing-queue ladder on-device.
- PUNKTFUNK_PRESENT_DEBUG gains latchMs p50/max (present-issue -> on-glass:
  standing queue reads ~n x period, healthy reads < 1 period) + noDrawable=.

swift test (14/14) + full Punktfunk-iOS device build green.

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

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

@@ -37,27 +37,34 @@ enum SettingsOptions {
     static let hudPlacements: [(label: String, tag: String)] =
         HUDPlacement.allCases.map { ($0.label, $0.rawValue) }
 
-    /// Stage-2 vs stage-3 present pacing (`DefaultsKey.presenter` — see SessionPresenter's
-    /// PresenterChoice); the freeze-prone stage-1 diagnostic only ships in DEBUG builds.
+    /// Present-pacing choices (`DefaultsKey.presenter` — see SessionPresenter's PresenterChoice):
+    /// stage-2 arrival, stage-3 glass-gated, stage-4 deadline (CAMetalDisplayLink — iOS/tvOS
+    /// only; macOS resolves it back to its default, so it isn't offered there). The freeze-prone
+    /// stage-1 diagnostic only ships in DEBUG builds.
     static var presenters: [(label: String, tag: String)] {
         var options: [(label: String, tag: String)] = [
             ("Stage 2", "stage2"),
             ("Stage 3", "stage3"),
         ]
+        #if !os(macOS)
+        options.append(("Stage 4", "stage4"))
+        #endif
         #if DEBUG
         options.append(("Stage 1 (debug)", "stage1"))
         #endif
         return options
     }
 
-    /// 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.
+    /// The platform's presenter default (mirrors SessionPresenter's platformDefault — iOS/iPadOS
+    /// runs deadline pacing, tvOS glass, macOS arrival). Views seed their @AppStorage display
+    /// from this so an untouched picker shows what actually runs.
     static var presenterDefault: String {
-        #if os(macOS)
-        "stage2"
-        #else
+        #if os(iOS)
+        "stage4"
+        #elseif os(tvOS)
         "stage3"
+        #else
+        "stage2"
         #endif
     }