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/StreamView.swift

@@ -165,6 +165,9 @@ public final class StreamLayerView: NSView {
     /// stage-1 StreamPump → displayLayer path as the Metal-unavailable / DEBUG fallback.
     private let presenter = SessionPresenter()
     public private(set) var connection: PunktfunkConnection?
+    /// 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 on every relayout.
+    private var matchFollower: MatchWindowFollower?
     private let cursorCapture = CursorCapture()
     private var inputCapture: InputCapture?
     private var appObservers: [NSObjectProtocol] = []
@@ -627,14 +630,28 @@ public final class StreamLayerView: NSView {
             makeDisplayLink: { displayLink(target: $0, selector: $1) },
             onFrame: onFrame,
             onSessionEnd: onSessionEnd)
+        // Match-window (C3): follow the window's pixel size when the setting is on. Latched at
+        // session start (mirrors the other clients); the first real `layout()` feeds the initial
+        // size, so the stream converges to the window even if the connect used the explicit mode.
+        matchFollower = MatchWindowFollower(
+            connection: connection,
+            enabled: UserDefaults.standard.bool(forKey: DefaultsKey.matchWindow))
         layoutPresenter()
         requestAutoCapture() // entering a session is the deliberate "capture me" moment
     }
 
     /// Aspect-fit the stage-2 metal sublayer to the view; refresh contentsScale on a
-    /// retina↔non-retina move (see SessionPresenter.layout).
+    /// retina↔non-retina move (see SessionPresenter.layout). Also feeds the Match-window follower
+    /// the view's physical-pixel size (bounds → backing), so a window resize / retina move follows.
     private func layoutPresenter() {
         presenter.layout(in: bounds, contentsScale: window?.backingScaleFactor ?? 1)
+        // Feed the follower only once in a window (backing scale is real then) and with real
+        // bounds — a pre-window layout would report point-sized dimensions.
+        if window != nil, bounds.width > 0, bounds.height > 0 {
+            let px = convertToBacking(bounds).size
+            matchFollower?.noteSize(
+                widthPx: Int(px.width.rounded()), heightPx: Int(px.height.rounded()))
+        }
     }
 
     public override func viewDidChangeBackingProperties() {
@@ -650,6 +667,7 @@ public final class StreamLayerView: NSView {
         inputCapture?.stop()
         inputCapture = nil
         presenter.stop()
+        matchFollower = nil
         connection = nil
     }