fix(apple/iOS): immersive streaming — edge-to-edge, no status bar, hidden cursor, native default mode

Streaming on iPad left the status bar up and the video boxed inside the safe areas, on
top of a 16:9 default mode letterboxing on the 4:3 screen, with the iPadOS cursor
hovering over the video. The session view is now immersive on iOS:

- .ignoresSafeArea + .statusBarHidden + .persistentSystemOverlays(.hidden) for the
  session only (home gets its chrome back on disconnect).
- First run seeds the stream mode from the device's native screen
  (UIScreen.nativeBounds + maximumFramesPerSecond) instead of 1920×1080 — verified
  live: a fresh install negotiated the iPad's 2752×2064 with the host. macOS keeps the
  1080p default (a desktop window is not the screen).
- The iPadOS cursor hides while over the video (UIPointerInteraction .hidden(),
  re-resolved on capture toggles) — the host renders its own cursor from our deltas;
  true pointer lock through UIHostingController remains the documented gap.

Found along the way (host-side, not fixed here): at very high modes a keyframe burst
can fill the UDP send buffer and m3 treats the sendmmsg WouldBlock as fatal
("session ended with error: submit_frame: WouldBlock") instead of backpressuring.

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

clients/apple/README.md

@@ -188,11 +188,14 @@ signing, bundle id `io.unom.punktfunk`. Notes:
    pickers are macOS-only). For the iPad-with-external-display setup: the target
    enables multiple scenes + indirect input events — on Stage Manager iPads, drag the
    punktfunk window onto the external screen and the stream runs there with full
-   keyboard/mouse/touch. Known gaps: `prefersPointerLocked` is declared on the stream
-   view controller but UIHostingController doesn't forward the preference from
-   representable children, so the system cursor stays visible (relative-mouse
-   forwarding works regardless — fixing it means putting the controller into the UIKit
-   presentation chain, e.g. a full-screen UIKit presentation on session start); and
+   keyboard/mouse/touch. While streaming the session is immersive (edge-to-edge,
+   status bar + home indicator hidden) and the iPadOS cursor is hidden over the video
+   (`UIPointerInteraction` `.hidden()` — visible again when ⌘⎋ releases capture); on
+   iOS first run the stream mode defaults to the device's native screen so the video
+   fills the display. Known gaps: true pointer LOCK (`prefersPointerLocked`) isn't
+   consulted through UIHostingController, so the hidden cursor can still drift onto a
+   second screen (fixing it means putting the controller into the UIKit presentation
+   chain); and
    AVAudioSession interruptions (calls, Siri) don't auto-restart the audio engines yet
    (reconnect recovers).