feat(resize/apple): Match-window mid-stream resize trigger + settings (C3)

design/midstream-resolution-resize.md Phase 2, Apple client. The stream mode
follows the session window/scene: a windowed macOS window resize or an iPad
Stage Manager / Split View scene change renegotiates the host's virtual
display + encoder via the existing PunktfunkConnection.requestMode, so a
windowed session streams native-resolution pixels instead of scaling.
Decode/present need nothing — VideoToolbox recreates its session on the
keyframe-derived format-description change (§1 table).

- MatchWindowFollower (new): the shared D2 trigger discipline — physical
  pixels even-floored + clamped ≥320×200, debounce to resize-end, ≥1 s
  between requests, skip a size equal to the live mode, request each distinct
  size at most once (stops re-asking a rejected size / looping on a host
  rollback). Pure normalize/request core is unit-tested (MatchWindowTests).
- macOS StreamLayerView: fed from layoutPresenter() (bounds → convertToBacking),
  guarded to once-in-a-window.
- iOS StreamViewController: fed from viewDidLayoutSubviews (bounds × render
  scale); iOS-only (iPhone fullscreen no-ops, tvOS uses AVDisplayManager).
- Settings: "Match window" toggle in the Stream mode section (iOS + macOS),
  DefaultsKey.matchWindow, read per session by the follower.

Verified on a Linux Swift 6.1.2 toolchain (the app target needs AppKit/UIKit,
unavailable here): the real MatchWindowFollower.swift type-checks in Swift 5
mode against a connection stub, and the pure discipline + the follower's
decision path pass a standalone harness (drag-settle + grow → exactly two
switches, refresh preserved, no re-request loop). A full build + on-device run
(macOS window, iPad Stage Manager) remains for a Mac.

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

clients/apple/Sources/PunktfunkClient/Settings/SettingsView+Sections.swift

@@ -13,6 +13,11 @@ extension SettingsView {
     // failed exactly one slice: the iOS archive (macOS/tvOS never compile that branch).
     @ViewBuilder var streamModeSection: some View {
         Section {
+            #if os(iOS) || os(macOS)
+            // Match-window (design/midstream-resolution-resize.md D1): follow the session
+            // window/scene, renegotiating the host mode on a resize. Off → the explicit mode below.
+            Toggle("Match window", isOn: $matchWindow)
+            #endif
             #if os(iOS)
             iosResolutionWheel
             iosRefreshRows
@@ -35,8 +40,11 @@ extension SettingsView {
         } header: {
             Text("Stream mode")
         } footer: {
-            Text("The host creates a virtual output at exactly this mode — "
-                + "native resolution, no scaling. \(Self.bitrateFooter)")
+            Text(matchWindow
+                ? "The stream follows this window — the host resizes its virtual output to match "
+                    + "as you resize, no scaling. \(Self.bitrateFooter)"
+                : "The host creates a virtual output at exactly this mode — "
+                    + "native resolution, no scaling. \(Self.bitrateFooter)")
                 .font(.geist(12, relativeTo: .caption))
                 .foregroundStyle(.secondary)
         }

clients/apple/Sources/PunktfunkClient/Settings/SettingsView.swift

@@ -21,6 +21,7 @@ struct SettingsView: View {
     @AppStorage(DefaultsKey.streamWidth) var width = 1920
     @AppStorage(DefaultsKey.streamHeight) var height = 1080
     @AppStorage(DefaultsKey.streamHz) var hz = 60
+    @AppStorage(DefaultsKey.matchWindow) var matchWindow = false
     @AppStorage(DefaultsKey.compositor) var compositor = 0
     @AppStorage(DefaultsKey.gamepadType) var gamepadType = 0
     @AppStorage(DefaultsKey.bitrateKbps) var bitrateKbps = 0