feat(apple): iPad ⌃⌥⇧Q release chord + click-to-recapture, pixel-grid snap, match-window opt-in

- InputCapture / StreamViewIOS: iPad ⌃⌥⇧Q un-capture chord, recognized from the
  GCKeyboard HID stream (no NSEvent monitor on iOS) for cross-client parity with
  the macOS/Windows/Linux combo; and a click into the video re-engages capture —
  the iPad analogue of macOS mouseDown → engageCapture(fromClick:), with the
  engaging click suppressed toward the host.
- SessionPresenter: snap the aspect-fit sublayer frame to the backing pixel grid.
  AVMakeRect centers the fit rect at fractional points, so the compositor
  resampled the layer — a uniform "everything soft" blur even when the drawable
  was pixel-exact 1:1. Rounding origin + size to device pixels makes the composite
  a true 1:1 blit; idempotent when already aligned.
- MetalVideoPresenter: PUNKTFUNK_BILINEAR_LUMA=1 A/B lever — compiles the shader
  with Catmull-Rom luma off (plain bilinear) to isolate bicubic overshoot from
  upstream fringing.
- SettingsView / StreamView / StreamViewIOS: match-window reverted to opt-in
  (default OFF) — the explicit mode is used and never auto-resized unless enabled.

clients/apple/Sources/PunktfunkKit/Views/StreamView.swift

@@ -661,15 +661,16 @@ public final class StreamLayerView: NSView {
                 DispatchQueue.main.async { self?.noteDecodedContentSize(width: w, height: h) }
                 overlayDecodedSize?(w, h)
             })
-        // Match-window (C3): follow the window's pixel size — DEFAULT ON, so a windowed session
-        // streams 1:1 (pixel-exact) instead of the presenter resampling a fixed-mode frame into a
+        // Match-window (C3): when ON, follow the window's pixel size so a windowed session streams
+        // 1:1 (pixel-exact) instead of the presenter resampling a fixed-mode frame into a
         // non-matching window. The first real `layout()` feeds the initial size, so the stream
         // converges to the window even though the connect used the explicit/display mode; entering
         // fullscreen reports the full-display px, restoring a native-res 1:1 present there too.
-        // `?? true` so an unset default matches the Settings toggle (which also defaults on).
+        // OPT-IN — `?? false` matches the Settings toggle (which also defaults off); an unset
+        // default keeps the explicit mode.
         let follower = MatchWindowFollower(
             connection: connection,
-            enabled: UserDefaults.standard.object(forKey: DefaultsKey.matchWindow) as? Bool ?? true)
+            enabled: UserDefaults.standard.object(forKey: DefaultsKey.matchWindow) as? Bool ?? false)
         follower.onResizeTarget = onResizeTarget // resize overlay START signal (instant, on the follower)
         matchFollower = follower
         layoutPresenter()