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.

CHANGELOG.md

@@ -0,0 +1,27 @@
+# Changelog
+
+Notable changes to `screen_cast`. Loosely follows [Keep a Changelog].
+
+## [Unreleased]
+
+### Added
+
+- **Phase 9 — iOS receiver app** (`ios/`): a native Swift receiver so an iPhone
+  can act as the second receiver. Speaks the existing signaling + RTP protocol
+  (no C++ changes), mirroring the Android receiver (Phase 8) source-to-source.
+  - RTP core (header/packet, jitter buffer, H.264 depacketizer) ported
+    source-to-source from the Android receiver.
+  - BSD-socket signaling server (dual-stack, most-recent-peer, never-throwing
+    sends) + `NSBonjourServices` advertisement.
+  - VideoToolbox H.264 decode (in-band SPS/PPS, real-time, session rebuilt on
+    resolution change) rendered via `AVSampleBufferDisplayLayer` (letterbox).
+  - PLI keyframe recovery (rate-limited 500 ms) and `pendingOffer` for late
+    surface attach; no placeholder sizing before the first real keyframe.
+  - XcodeGen project + `bootstrap.sh`; XCTest port of the Android suite plus new
+    signaling / line-framing / AVCC / NAL-extraction coverage.
+
+> **Status:** authored; on-device validation pending a Mac + Xcode 26 + iPhone 16
+> (local-network/Bonjour consent, the H.264 format-description recipe, and the
+> VideoToolbox render path are the on-device verification items).
+
+[Keep a Changelog]: https://keepachangelog.com/en/1.1.0/