feat(ios): native receiver app (Swift, min iOS 17)

A native iOS receiver so an iPhone can act as the second receiver, speaking the existing signaling + RTP protocol (no C++ changes) and mirroring the Android receiver (Phase 8) source-to-source.

- RTP core (header/packet, jitter buffer, H.264 depacketizer) ported from the Android receiver
- BSD-socket signaling server (dual-stack, most-recent-peer, never-throwing sends) + NSBonjourServices
- VideoToolbox H.264 decode (in-band SPS/PPS, real-time, rebuilds on size change) -> AVSampleBufferDisplayLayer
- PLI keyframe recovery (500 ms) + pendingOffer for late surface attach
- XcodeGen project + bootstrap.sh; XCTest port of the Android suite + new coverage
- .gitignore for generated artifacts; CHANGELOG; PHASES + MEMORY updated

Status: authored; on-device validation pending a Mac + Xcode 26 + iPhone 16.

.agents/MEMORY.md

@@ -7,6 +7,50 @@ bottom of this file.
 
 ## Project state
 
+- **Phase 9 in progress: iOS receiver app** (`ios/`, Swift, min iOS 17):
+  iPhone as a second receiver. Speaks the same signaling+RTP protocol — no C++
+  changes. Mirrors the Android receiver (Phase 8) source-to-source.
+  - Scaffolding: XcodeGen `project.yml` (Info.plist carries
+    `NSLocalNetworkUsageDescription` + `NSBonjourServices: _screencast._tcp`) +
+    `bootstrap.sh` (downloads XcodeGen from the GitHub release, no Homebrew;
+    generates the project; builds/tests via xcodebuild). `ios/README.md` has
+    build/install/device steps.
+  - App (`Receiver/App`): SwiftUI + `AVSampleBufferDisplayLayer` (`.resizeAspect`
+    = letterbox — the same "size the surface, not a transform" lesson as
+    Android); `ReceiverController` (ObservableObject) drives start/stop on
+    scenePhase.
+  - Protocol core (`Receiver/Rtp`, `Receiver/Signaling`): RtpHeader/RtpPacket/
+    JitterBuffer/H264Depacketizer ported source-to-source; SignalingMessage
+    (JSONSerialization) + LineAssembler + SignalingServer (BSD sockets,
+    dual-stack, most-recent-peer-wins, MSG_NOSIGNAL sends that never throw);
+    `AvccConverter` (Annex-B ↔ AVCC) + `NalExtractor` (SPS/PPS) are new for the
+    VideoToolbox path.
+  - Decode (`Receiver/Decode`): `H264FormatDescription` (Core Foundation H.264
+    config recipe) + `H264VideoToolboxDecoder` (in-band SPS/PPS → session;
+    `kVTDecompressionPropertyKey_RealTime`; session recreated on size change;
+    the output callback may run on a worker thread, so state is lock-guarded)
+    → `AVSampleBufferRenderSink` (AVSampleBufferDisplayLayerSession).
+  - Support (`Receiver/Support`): `UdpTransport` (poll-based recv so close() can't
+    strand a blocked recvfrom) + `LocalAddress` (getifaddrs for the --peer hint).
+  - Pipeline (`Receiver/Pipeline`): ReceiverPipeline — one serial queue for all
+    state + decode; reader thread only polls/receives UDP; pendingOffer for late
+    surface attach (+ PLI); PLI rate-limited 500 ms; first-frame flag; video
+    size tracked (never reports a placeholder before the first real keyframe —
+    the Android startup-squish lesson).
+  - Tests (`ios/ReceiverTests`): the 25 Android JVM tests ported to XCTest plus
+    new coverage for signaling JSON, line framing, AVCC, NAL extraction.
+  - `.gitignore` excludes the generated `ios/Receiver.xcodeproj/`, `ios/tools/`,
+    `ios/Receiver/Info.plist` (the project.yml `info` block is the source of
+    truth).
+  - **NOT YET VALIDATED** (this box is Linux, no Xcode/iOS SDK): nothing here
+    compiles or runs. 9.5 needs a Mac with Xcode 26 + the iPhone 16.
+    Highest-risk on-device items: (1) local-network + Bonjour consent
+    (NSBonjourServices must be `_screencast._tcp`; iOS 26 tightened the prompt),
+    (2) the H264FormatDescription CF ownership recipe (a bug crashes on the
+    first keyframe — loud, not silent), (3) VideoToolbox decode →
+    AVSampleBufferDisplayLayer render. Run `ios/bootstrap.sh test` first on the
+    Mac.
+
 - **Phase 8 done: Android receiver app** (`android/`, Kotlin, minSdk 30,
   app id `screen_cast.receiver`): phone as second receiver (screen → HDMI via
   USB-C DP-alt-mode). Speaks the existing signaling+RTP protocol — no C++