Initial WebhookServer implementation

Add the .NET 8 solution scaffolded against PLAN.md. Three projects share
WebhookServer.Core (models, auth, execution, storage, IPC, callbacks)
and WebhookServer.Service hosts an embedded Kestrel listener plus the
named-pipe admin server. WebhookServer.Gui is a thin MVVM client over
the pipe. Includes 25 unit tests covering HMAC verification, bearer
auth, IP allowlist parsing, arg-template rendering, DPAPI round-trip,
and the encrypt-on-save config store.

Install/uninstall PowerShell scripts default to LocalSystem and accept
a domain user or gMSA via -ServiceAccount.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Core/Auth/IpAllowList.cs

@@ -0,0 +1,87 @@
+using System.Net;
+using System.Net.Sockets;
+
+namespace WebhookServer.Core.Auth;
+
+/// <summary>
+/// Compiled allow-list of IPs and CIDR ranges. Empty list = allow all.
+/// </summary>
+public sealed class IpAllowList
+{
+    private readonly List<IPNetwork> _networks;
+
+    public bool IsEmpty => _networks.Count == 0;
+
+    private IpAllowList(List<IPNetwork> networks) => _networks = networks;
+
+    public bool Contains(IPAddress address)
+    {
+        if (IsEmpty) return true;
+
+        var normalized = Normalize(address);
+        foreach (var net in _networks)
+        {
+            if (net.BaseAddress.AddressFamily != normalized.AddressFamily) continue;
+            if (net.Contains(normalized)) return true;
+        }
+        return false;
+    }
+
+    /// <summary>
+    /// Parse a list of allowlist entries. Each entry may be a single IP or a CIDR.
+    /// Throws <see cref="FormatException"/> on the first invalid entry.
+    /// </summary>
+    public static IpAllowList Parse(IEnumerable<string> entries)
+    {
+        var nets = new List<IPNetwork>();
+        foreach (var raw in entries)
+        {
+            var entry = raw?.Trim();
+            if (string.IsNullOrEmpty(entry)) continue;
+            nets.Add(ParseEntry(entry));
+        }
+        return new IpAllowList(nets);
+    }
+
+    public static bool TryParse(IEnumerable<string> entries, out IpAllowList list, out string? error)
+    {
+        var nets = new List<IPNetwork>();
+        foreach (var raw in entries)
+        {
+            var entry = raw?.Trim();
+            if (string.IsNullOrEmpty(entry)) continue;
+            try
+            {
+                nets.Add(ParseEntry(entry));
+            }
+            catch (FormatException ex)
+            {
+                list = new IpAllowList(new List<IPNetwork>());
+                error = $"invalid entry '{raw}': {ex.Message}";
+                return false;
+            }
+        }
+        list = new IpAllowList(nets);
+        error = null;
+        return true;
+    }
+
+    private static IPNetwork ParseEntry(string entry)
+    {
+        if (entry.Contains('/'))
+            return IPNetwork.Parse(entry);
+
+        if (!IPAddress.TryParse(entry, out var addr))
+            throw new FormatException($"'{entry}' is not a valid IP address or CIDR");
+
+        var prefix = addr.AddressFamily == AddressFamily.InterNetworkV6 ? 128 : 32;
+        return new IPNetwork(Normalize(addr), prefix);
+    }
+
+    private static IPAddress Normalize(IPAddress address)
+    {
+        if (address.AddressFamily == AddressFamily.InterNetworkV6 && address.IsIPv4MappedToIPv6)
+            return address.MapToIPv4();
+        return address;
+    }
+}