Allow public IPv4 addresses for direct SRTP calls

Use shared unicast IPv4 validation for interface discovery, media endpoints and server signaling. Preserve disclosure consent and privacy constraints, and document direct connectivity requirements.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/PostIt/PostIt.Tests/LanCallViewModelTests.cs

@@ -114,6 +114,18 @@ public sealed class LanCallViewModelTests
         return vm;
     }
 
+    [AvaloniaFact]
+    public async Task Public_ipv4_can_be_selected_for_an_explicitly_consented_direct_call()
+    {
+        var signaling = new Signaling();
+        await using var vm = Configure(signaling, (_, _) => throw new Exception("No media before acceptance"));
+        vm.Addresses.Add("8.8.8.8");
+        vm.Address = "8.8.8.8";
+        await vm.InviteCommand.ExecuteAsync(null);
+        Assert.Equal(1, signaling.Invites);
+        Assert.True(vm.Busy);
+    }
+
     [AvaloniaFact]
     public async Task No_capture_starts_before_explicit_acceptance_and_endpoint_exchange()
     {