feat(apple): tabbed macOS Settings + stats-overlay placement/toggle + Stream menu

The macOS Settings window had outgrown one scrolling pane — split it into a tabbed
preferences window (General / Display / Audio / Controllers / Advanced). Each
settings group is now a shared @ViewBuilder section, so iOS keeps its single
grouped Form and tvOS its pushed-picker layout, each defined once. No setting
moved or dropped.

New statistics-overlay controls (Settings → Display → Statistics): a show/hide
toggle (DefaultsKey.hudEnabled) and a corner picker (HUDPlacement /
DefaultsKey.hudPlacement) — the HUD moves to the chosen corner and aligns its text
to that edge.

A Scene-level "Stream" menu (StreamCommands) carries Show/Hide Statistics (⌘⇧S)
and Disconnect (⌘D). Disconnect moved off the HUD button into the menu so it
survives the overlay being hidden, wired via .focusedSceneValue. On iOS a
material-backed exit chip appears when the HUD is hidden (touch users have no
menu/⌘D); tvOS disconnect is unchanged (Siri-Remote Menu button).

Builds on macOS/iOS/tvOS; swift test green. Adversarially reviewed (8 findings
refuted, 2 minor — the iOS exit-chip contrast fix is included here).

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

clients/apple/Sources/PunktfunkKit/DefaultsKeys.swift

@@ -26,4 +26,10 @@ public enum DefaultsKey {
     public static let libraryEnabled = "punktfunk.libraryEnabled"
     /// macOS: take the window fullscreen while streaming and restore it on the host list. On by default.
     public static let fullscreenWhileStreaming = "punktfunk.fullscreenWhileStreaming"
+    /// Show the streaming statistics overlay (mode/fps/throughput/latency). On by default; toggle
+    /// while streaming with ⌘⇧S (macOS / hardware keyboard).
+    public static let hudEnabled = "punktfunk.hudEnabled"
+    /// Which corner the statistics overlay sits in — a `HUDPlacement` raw value
+    /// ("topLeading"/"topTrailing"/"bottomLeading"/"bottomTrailing"). Default top-trailing.
+    public static let hudPlacement = "punktfunk.hudPlacement"
 }