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/SessionModel.swift

@@ -207,9 +207,9 @@ final class SessionModel: ObservableObject {
         let defaults = UserDefaults.standard
         let audio = SessionAudio(connection: conn)
         audio.start(
-            speakerUID: defaults.string(forKey: "punktfunk.speakerUID") ?? "",
-            micUID: defaults.string(forKey: "punktfunk.micUID") ?? "",
-            micEnabled: defaults.object(forKey: "punktfunk.micEnabled") as? Bool ?? true)
+            speakerUID: defaults.string(forKey: DefaultsKey.speakerUID) ?? "",
+            micUID: defaults.string(forKey: DefaultsKey.micUID) ?? "",
+            micEnabled: defaults.object(forKey: DefaultsKey.micEnabled) as? Bool ?? true)
         self.audio = audio
         // Gamepads: forward GamepadManager's active controller as pad 0 and render the
         // host's feedback (rumble always; lightbar/player-LEDs/adaptive-triggers when the