feat: add loopback tunnel configuration primitives

StikJIT/Tunnel/IPv4PacketRewriter.swift

@@ -0,0 +1,16 @@
+//
+//  IPv4PacketRewriter.swift
+//  StikDebug
+//
+
+enum IPv4PacketRewriter {
+    static func swapEndpoints(in packet: inout [UInt8]) {
+        guard packet.count >= 20, packet[0] >> 4 == 4 else {
+            return
+        }
+
+        for offset in 0..<4 {
+            packet.swapAt(12 + offset, 16 + offset)
+        }
+    }
+}

StikJIT/Tunnel/StikDebugTunnelConfiguration.swift

@@ -0,0 +1,31 @@
+//
+//  StikDebugTunnelConfiguration.swift
+//  StikDebug
+//
+
+struct StikDebugTunnelConfiguration: Equatable {
+    static let interfaceIPKey = "interfaceIP"
+    static let peerIPKey = "peerIP"
+
+    static let `default` = StikDebugTunnelConfiguration(
+        interfaceIP: "10.7.0.0",
+        peerIP: "10.7.0.1"
+    )
+
+    let interfaceIP: String
+    let peerIP: String
+    let peerPrefixLength: Int
+
+    init(interfaceIP: String, peerIP: String, peerPrefixLength: Int = 32) {
+        self.interfaceIP = interfaceIP
+        self.peerIP = peerIP
+        self.peerPrefixLength = peerPrefixLength
+    }
+
+    var providerConfiguration: [String: String] {
+        [
+            Self.interfaceIPKey: interfaceIP,
+            Self.peerIPKey: peerIP,
+        ]
+    }
+}

StikJITTests/StikJITTests.swift

@@ -11,6 +11,36 @@ import Testing
 
 struct StikJITTests {
 
+    @Test func embeddedTunnelDefaultsMatchLocalDevVPNEndpoint() {
+        let configuration = StikDebugTunnelConfiguration.default
+
+        #expect(configuration.interfaceIP == "10.7.0.0")
+        #expect(configuration.peerIP == "10.7.0.1")
+        #expect(configuration.peerPrefixLength == 32)
+        #expect(configuration.providerConfiguration["interfaceIP"] == "10.7.0.0")
+        #expect(configuration.providerConfiguration["peerIP"] == "10.7.0.1")
+    }
+
+    @Test func ipv4PacketRewriterSwapsPacketEndpoints() {
+        var packet = [UInt8](repeating: 0, count: 20)
+        packet.replaceSubrange(12..<16, with: [10, 7, 0, 2])
+        packet.replaceSubrange(16..<20, with: [10, 7, 0, 1])
+
+        IPv4PacketRewriter.swapEndpoints(in: &packet)
+
+        #expect(Array(packet[12..<16]) == [10, 7, 0, 1])
+        #expect(Array(packet[16..<20]) == [10, 7, 0, 2])
+    }
+
+    @Test func ipv4PacketRewriterLeavesShortPacketsUnchanged() {
+        var packet = [UInt8](repeating: 0, count: 19)
+        let original = packet
+
+        IPv4PacketRewriter.swapEndpoints(in: &packet)
+
+        #expect(packet == original)
+    }
+
     @Test func txmDetectionIgnoresFirmwareFileBeforeIOS26() async throws {
         let isSupported = ProcessInfo.hasTXMSupport(
             operatingSystemVersion: OperatingSystemVersion(majorVersion: 18, minorVersion: 7, patchVersion: 2),