Discovery: wide-area bridge DNS-SD

# Conflicts:
#	apps/ios/Sources/Bridge/BridgeDiscoveryModel.swift
#	src/cli/dns-cli.ts

apps/macos/Sources/Clawdis/InstanceIdentity.swift

@@ -1,3 +1,4 @@
+import Darwin
 import Foundation
 
 enum InstanceIdentity {
@@ -30,4 +31,15 @@ enum InstanceIdentity {
         }
         return "clawdis-mac"
     }()
+
+    static let modelIdentifier: String? = {
+        var size = 0
+        guard sysctlbyname("hw.model", nil, &size, nil, 0) == 0, size > 1 else { return nil }
+
+        var buffer = [CChar](repeating: 0, count: size)
+        guard sysctlbyname("hw.model", &buffer, &size, nil, 0) == 0 else { return nil }
+
+        let s = String(cString: buffer).trimmingCharacters(in: .whitespacesAndNewlines)
+        return s.isEmpty ? nil : s
+    }()
 }