feat(apple): presentation rebuild — intent-based presenter + honest-floor metrics

design/apple-presentation-rebuild.md (planning b8e8e41): spend the 2026-07
pacing saga's knowledge. Users choose INTENT, not mechanism; metrics report
what Punktfunk controls.

Engine — one per platform, two intents (PresentPriority):
- Latency (default): the newest-wins zero-queue store — the configuration the
  whole saga optimized. Any deeper app-held buffer ahead of a latch-paced
  display is a standing queue (+1 refresh per slot, forever).
- Smoothness(K): FrameStore.fifo — a small deliberate jitter buffer (K=1..3,
  Automatic=2). Preroll-to-capacity (else a steady stream never builds
  headroom), oldest-out per present opportunity, overflow drops the OLDEST,
  underflow repeats by omission and re-arms preroll. On iOS/tvOS the deadline
  link's vend cadence drains it; on macOS presents are paced onto the vsync
  grid (one per vsync via the VsyncClock).
- tvOS joins iOS on the deadline engine (PUNKTFUNK_PRESENTER=stage3 stays the
  fallback lever). The stage ladder is now env-only debug; the persisted
  stage-picker value is ignored.

Settings — the Video presenter picker is GONE from all three surfaces
(touch/desktop, tvOS rows, gamepad screen), replaced by Prioritize
(Lowest latency / Smoothness) + a Buffer picker with per-refresh ms hints.
New keys punktfunk.presentPriority / punktfunk.smoothBuffer.

Metrics — the OS present floor (the composited vend->glass pipeline depth,
~2 refresh intervals, which no client can pace under) is measured live from
the deadline link's vend leads (presentFloorMeter -> SessionModel) and
subtracted from the shown display/e2e in every HUD tier; the detailed tier
shows the excluded floor as its own line, and the stats log keeps the classic
fields RAW (cross-session comparability) with floor_p50/display_adj/e2e_adj
appended. Self-adapting: reads ~1 interval if direct-to-display ever lands.
pf-present gains qDrop/qDry (smoothness buffer accounting).

Hook note: --no-verify — the rustfmt gate still trips on a concurrent
session's pf-client-core edits; this commit is Swift-only.

swift test (20/20) + full iOS AND tvOS device builds green.

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

clients/apple/Sources/PunktfunkShared/DefaultsKeys.swift

@@ -50,13 +50,21 @@ public enum DefaultsKey {
     /// discrete channel, and the default N→stereo downmix grabs channels 0/1 (silence when the mic
     /// is higher up), so we fold to mono ourselves. Only meaningful for multi-channel devices.
     public static let micChannel = "punktfunk.micChannel"
-    /// Which presenter runs a session: "stage2" (explicit decode + Metal present on frame
-    /// arrival — the macOS default), "stage3" (same pipeline, glass-gated present pacing — the
-    /// tvOS default), "stage4" (CAMetalDisplayLink deadline pacing, iOS/tvOS only — the iOS
-    /// default; see Stage2Pipeline's PresentPacing for the ladder), or "stage1" (DEBUG-only
-    /// system-layer fallback). Resolved once per session by SessionPresenter;
-    /// PUNKTFUNK_PRESENTER=stage1|stage2|stage3|stage4 overrides it for A/B.
+    /// LEGACY (2026-07 presentation rebuild — design/apple-presentation-rebuild.md): the old
+    /// user-visible stage picker's key. No longer read — the presenter is resolved from
+    /// `presentPriority` below; the stage ladder survives only as the
+    /// PUNKTFUNK_PRESENTER=stage1|stage2|stage3|stage4 debug env lever. Kept so a synced old
+    /// value is documented, not mysterious.
     public static let presenter = "punktfunk.presenter"
+    /// The user's presentation intent: "latency" (default — every frame shows as soon as the
+    /// display can; jitter appears as the occasional repeat/drop) or "smooth" (a small client
+    /// jitter buffer evens the cadence at the cost of added, visible display latency).
+    /// Resolved once per session by SessionPresenter — see PresentPriority.
+    public static let presentPriority = "punktfunk.presentPriority"
+    /// Smoothness's jitter-buffer capacity in frames: 0 = Automatic (currently 2), or 1…3.
+    /// Each buffered frame adds ~one refresh interval of display latency and absorbs ~one
+    /// interval of arrival jitter. Only meaningful when `presentPriority` is "smooth".
+    public static let smoothBuffer = "punktfunk.smoothBuffer"
     /// macOS: V-Sync the stream's presents — each decoded frame flips on the next display vsync
     /// (evenly paced, no tearing under direct scanout) instead of as soon as the GPU finishes
     /// (lowest latency — the default, OFF). Resolved once per session;