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/Package.swift

@@ -0,0 +1,24 @@
+// swift-tools-version: 6.0
+// T-iOS-1 scaffold shell. SessionEngine / ReconnectMachine / PingScheduler /
+// GateState / AwayDigest / URLSessionTermTransport land in W1–W2 tasks.
+// Dependency direction is strictly downward: SessionCore → WireProtocol only.
+import PackageDescription
+
+let package = Package(
+    name: "SessionCore",
+    platforms: [.iOS(.v17), .macOS(.v14)],
+    products: [
+        .library(name: "SessionCore", targets: ["SessionCore"]),
+    ],
+    dependencies: [
+        .package(path: "../WireProtocol"),
+    ],
+    targets: [
+        .target(
+            name: "SessionCore",
+            dependencies: [.product(name: "WireProtocol", package: "WireProtocol")]
+        ),
+        .testTarget(name: "SessionCoreTests", dependencies: ["SessionCore"]),
+    ],
+    swiftLanguageModes: [.v6]
+)