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/HostStore.swift

@@ -9,6 +9,7 @@
 // --require-pairing only admit paired clients, so for them pairing is the only way in.
 
 import Foundation
+import PunktfunkKit
 import SwiftUI
 
 struct StoredHost: Identifiable, Codable, Hashable {
@@ -26,7 +27,7 @@ struct StoredHost: Identifiable, Codable, Hashable {
 
 @MainActor
 final class HostStore: ObservableObject {
-    private static let key = "punktfunk.hosts"
+    private static let key = DefaultsKey.hosts
 
     @Published var hosts: [StoredHost] {
         didSet { persist() }