feat(apple): stage-3 presenter — glass-gated present pacing as a live A/B

Stage-2's present-on-arrival saturates CAMetalLayer's FIFO image queue
whenever the stream rate runs at the panel's refresh (iOS always
vsync-latches; the macOS 26 compositor latch-paces our out-of-band
presents the same way): one early burst fills the queue to
maximumDrawableCount and — arrivals then matching latches one-for-one —
it never drains. That sticky depth is the measured 29-30 ms display
stage on the 120 Hz ProMotion devices, and the full-queue regime is
where host<->panel clock drift turns into the reported fixed-interval
repeats/drops. The 240 Hz Studio never saturates, which is why it never
showed either symptom.

Stage-3 is the same pipeline with a PresentGate: at most ONE
presented-but-undisplayed drawable in flight; the drawable's presented
handler reopens the gate and re-signals the render thread, so the next
present always takes the freshest newest-wins ring frame — the hidden
queue latency becomes explicit, correct frame drops. A 100 ms stale
fallback force-opens a gate whose handler never fires (the "presents
aren't damage" hazard class) so a pathological system degrades visibly
instead of freezing; the PUNKTFUNK_PRESENT_DEBUG `forced` counter
exposes it (0 on healthy systems).

Selection: the Settings > Display presenter picker now ships in release
builds (stage 2 default / stage 3 experimental; the freeze-prone
stage-1 diagnostic stays DEBUG-only), resolved per session with a
PUNKTFUNK_PRESENTER=stage1|stage2|stage3 env override for CLI A/B. The
pf-present debug line gains gated/forced/inflightMax — inflightMax is
the direct image-queue-depth measurement for the A/B.

Live-verified both ways against a real host at 1080p120: stage-3 holds
120/120 fps with inflightMax=1, forced=0, glass deltas p50 8.33 ms;
stage-2 is behaviorally unchanged (120/120 fps, inflightMax=2 even on
the wired 240 Hz setup — the saturation signal in miniature). Unit
tests cover the gate (one-in-flight, stale force-open, idempotent
release) and the presenter resolution (env override, release stage-1
gating); macOS tests green, iOS/tvOS xcodebuild clean.

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

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

@@ -284,6 +284,19 @@ struct SettingsView: View {
         ("4K @ 60", "3840x2160x60"),
     ]
 
+    /// Stage-2 vs stage-3 present pacing (see SettingsView+Sections' presenterSection for the
+    /// rationale); the freeze-prone stage-1 diagnostic only ships in DEBUG builds.
+    private static var presenterOptions: [(label: String, tag: String)] {
+        var options: [(label: String, tag: String)] = [
+            ("Stage 2 (default)", "stage2"),
+            ("Stage 3 (experimental)", "stage3"),
+        ]
+        #if DEBUG
+        options.append(("Stage 1 (debug)", "stage1"))
+        #endif
+        return options
+    }
+
     private var modeTag: Binding<String> {
         Binding(
             get: { "\(width)x\(height)x\(hz)" },
@@ -332,12 +345,10 @@ struct SettingsView: View {
                 TVSelectionRow(
                     title: "Compositor", options: SettingsOptions.compositors,
                     selection: $compositor)
-                #if DEBUG
                 TVSelectionRow(
-                    title: "Presenter (debug)",
-                    options: [("Stage 2 (default)", "stage2"), ("Stage 1 (debug)", "stage1")],
+                    title: "Presenter",
+                    options: Self.presenterOptions,
                     selection: $presenter)
-                #endif
                 TVSelectionRow(
                     title: "10-bit HDR",
                     options: [("On", "on"), ("Off", "off")], selection: hdrEnabledTag)