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/PunktfunkKit/Views/StreamViewIOS.swift

@@ -147,6 +147,11 @@ public final class StreamViewController: StreamViewControllerBase {
     /// Capture state at the last resign, restored on the next foreground — otherwise the
     /// mouse/keyboard stay released after navigating out and nothing re-grabs them.
     private var wasCapturedOnResign = false
+    /// Match-window resize follower (C3) — non-nil while a session is active AND the `matchWindow`
+    /// setting is on; fed the view's physical-pixel size from `viewDidLayoutSubviews` so an iPad
+    /// Stage Manager / Split View scene resize renegotiates the host mode. iOS only (iPhone
+    /// naturally no-ops fullscreen; tvOS drives display modes via AVDisplayManager instead).
+    private var matchFollower: MatchWindowFollower?
     #endif
 
     /// Reads whether the scene's pointer is actually locked right now; nil = state
@@ -327,6 +332,12 @@ public final class StreamViewController: StreamViewControllerBase {
         }
         capture.start()
         inputCapture = capture
+        // Match-window (C3): follow the scene's pixel size when the setting is on. Latched at
+        // session start (mirrors the other clients); `viewDidLayoutSubviews` feeds it — covers
+        // Stage Manager / Split View resizes and rotation. iPhone fullscreen naturally no-ops.
+        matchFollower = MatchWindowFollower(
+            connection: connection,
+            enabled: UserDefaults.standard.bool(forKey: DefaultsKey.matchWindow))
         #endif
 
         // Presenter choice + lifecycle live in SessionPresenter (shared with macOS): stage-2
@@ -411,6 +422,7 @@ public final class StreamViewController: StreamViewControllerBase {
         streamView.onPointerButton = nil
         streamView.onScroll = nil
         streamView.currentHostMode = nil
+        matchFollower = nil
         #endif
         #if os(tvOS)
         // Return the TV to the user's preferred mode — the home screen must not stay in the
@@ -425,6 +437,16 @@ public final class StreamViewController: StreamViewControllerBase {
     public override func viewDidLayoutSubviews() {
         super.viewDidLayoutSubviews()
         layoutMetalLayer()
+        #if os(iOS)
+        // Match-window (C3): feed the follower the view's physical-pixel size (points × scale).
+        let b = streamView.bounds
+        if b.width > 0, b.height > 0 {
+            let scale = renderScale
+            matchFollower?.noteSize(
+                widthPx: Int((b.width * scale).rounded()),
+                heightPx: Int((b.height * scale).rounded()))
+        }
+        #endif
         #if os(tvOS)
         applyDisplayCriteriaIfNeeded()
         #endif