feat(ios): W0 scaffold + day-1 spike + WireProtocol frozen contract + TestSupport doubles

T-iOS-1: ios/ XcodeGen project (iOS 17, Swift 6 strict concurrency, ATS per PLAN §5.2), 5 SPM package shells, CI skeleton
T-iOS-2: Origin spike vs real server — URLSessionWebSocketTask custom Origin CONFIRMED (no Starscream); 16MiB replay + EMSGSIZE(40) errno correction written back to plan
T-iOS-3: WireProtocol frozen contract, 59 tests, 100% line coverage, cross-impl vectors vs src/protocol.ts via tsx
T-iOS-4: FakeTransport/FakeClock/FakeHTTPTransport doubles
Verify: independent agent re-ran all acceptance — 6/6 PASS

ios/Packages/SessionCore/Sources/SessionCore/SessionCorePlaceholder.swift

@@ -0,0 +1,13 @@
+import WireProtocol
+
+/// T-iOS-1 scaffold placeholder.
+///
+/// Real modules land in their owning tasks: ReconnectMachine + PingScheduler
+/// (T-iOS-5), GateState + AwayDigest (T-iOS-6), URLSessionTermTransport
+/// (T-iOS-9), SessionEngine + SessionEvent (T-iOS-10), KeyByteMap (T-iOS-11).
+public enum SessionCorePackage {
+    /// Package identity marker consumed by the scaffold smoke test.
+    public static let packageName = "SessionCore"
+    /// Proves the single allowed dependency edge (SessionCore → WireProtocol) compiles.
+    public static let contractPackageName = WireProtocolPackage.packageName
+}