refactor(apple): code-quality pass — audit fixes + centralized defaults keys

A 6-agent adversarial audit of the client (11 confirmed of 39 findings, the rest
filtered) drove these:

- fix: SessionAudio ring buffer — guard a write larger than the ring (would push
  readIdx past writeIdx and corrupt the buffer; never happens, but guard not corrupt).
- fix: CADisplayLink retain cycle (stage-2 presenter) — a weak-target DisplayLinkProxy
  so the view can deallocate (the link retains its target); stage-2 teardown added to
  both StreamView/StreamViewController deinits as a safety net.
- fix: GamepadFeedback deinit { flag.stop() } — the drain thread holds the connection
  strongly and self weakly, so an abrupt teardown without stop() would leak it.
- refactor: centralize the 12 UserDefaults/@AppStorage key literals (scattered across
  8 files) into one DefaultsKey enum — a typo silently splits a setting's reader from
  its writer.
- docs: RumbleRenderer @unchecked Sendable invariant; the HID digit-row table; the
  stage-2 layer compositing.

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

clients/apple/Sources/PunktfunkClient/SpeedTestSheet.swift

@@ -32,10 +32,10 @@ struct SpeedTestSheet: View {
     @Environment(\.dismiss) private var dismiss
     let host: StoredHost
 
-    @AppStorage("punktfunk.width") private var width = 1920
-    @AppStorage("punktfunk.height") private var height = 1080
-    @AppStorage("punktfunk.hz") private var hz = 60
-    @AppStorage("punktfunk.bitrateKbps") private var bitrateKbps = 0
+    @AppStorage(DefaultsKey.streamWidth) private var width = 1920
+    @AppStorage(DefaultsKey.streamHeight) private var height = 1080
+    @AppStorage(DefaultsKey.streamHz) private var hz = 60
+    @AppStorage(DefaultsKey.bitrateKbps) private var bitrateKbps = 0
 
     private enum Phase: Equatable {
         case connecting