docs and networking: improve viewport reach detection

Multiplex/Models/ViewportReach.swift

@@ -56,6 +56,7 @@ enum ViewportReach: Equatable {
         if host == "localhost" || host.hasSuffix(".localhost") { return true }
         if host == "0.0.0.0" || host == "::" || host == "::1" { return true }
         if let octets = ipv4Octets(host) { return octets[0] == 127 }
+        if let octets = ipv4MappedOctets(host) { return octets[0] == 127 }
         return false
     }
 
@@ -67,6 +68,13 @@ enum ViewportReach: Equatable {
             if octets[0] == 169, octets[1] == 254 { return true }
             return false
         }
+        if let octets = ipv4MappedOctets(host) {
+            if octets[0] == 10 { return true }
+            if octets[0] == 192, octets[1] == 168 { return true }
+            if octets[0] == 172, (16...31).contains(octets[1]) { return true }
+            if octets[0] == 169, octets[1] == 254 { return true }
+            return false
+        }
         // IPv6 unique-local (fc00::/7) and link-local (fe80::/10).
         if host.hasPrefix("fc") || host.hasPrefix("fd") || host.hasPrefix("fe80:") {
             return host.contains(":")
@@ -78,6 +86,14 @@ enum ViewportReach: Equatable {
         return false
     }
 
+    /// The IPv4 payload of the common `::ffff:w.x.y.z` mapped form, or nil.
+    private static func ipv4MappedOctets(_ host: String) -> [Int]? {
+        guard let separator = host.lastIndex(of: ":"),
+              host[..<separator] == "::ffff"
+        else { return nil }
+        return ipv4Octets(String(host[host.index(after: separator)...]))
+    }
+
     /// The four octets of a dotted-quad IPv4 literal, or nil for anything
     /// else (names, IPv6, malformed quads).
     private static func ipv4Octets(_ host: String) -> [Int]? {

MultiplexTests/ViewportReachTests.swift

@@ -38,6 +38,13 @@ final class ViewportReachTests: XCTestCase {
         }
     }
 
+    func testIPv4MappedPrivateAddressesKeepTheirIPv4Reach() {
+        XCTAssertEqual(classify("http://[::ffff:192.168.1.68]:5173/"), .lan)
+        XCTAssertEqual(classify("http://[::ffff:10.0.0.5]/"), .lan)
+        XCTAssertEqual(classify("http://[::ffff:127.0.0.1]:8080/"), .remoteLoopback)
+        XCTAssertEqual(classify("http://[::ffff:8.8.8.8]/"), .internet)
+    }
+
     func testUnqualifiedSingleLabelIsLAN() {
         // A bare machine name resolves through the local network's search
         // domains — LAN by construction.

README.md

@@ -55,7 +55,13 @@ Some features require Multiplex Pro; see [`docs/store-metadata.md`](docs/store-m
 
 ## Building
 
-Requires Xcode with the visionOS SDK and [XcodeGen](https://github.com/yonaskolb/XcodeGen). The project is generated, so edit `project.yml`, not the `.xcodeproj`.
+Requires Xcode with the visionOS SDK and [XcodeGen](https://github.com/yonaskolb/XcodeGen). Install the command-line prerequisites with [Homebrew](https://brew.sh/):
+
+```sh
+brew install xcodegen swiftlint
+```
+
+The Xcode project is generated, so edit `project.yml`, not the `.xcodeproj`. Regenerate it after project configuration changes:
 
 ```sh
 xcodegen generate