feat(pdu): add PDU MCP server for AI-driven SNMP power control

New ETTerms.PduMcp stdio MCP server lets AI agents (Kiro/Claude CLI)
control SNMP PDU outlets directly. Unlike serial, SNMP is non-exclusive
so it talks to the PDU directly without bridging through the GUI.

Tools: pdu_connect / pdu_list / pdu_set_port / pdu_get_port / pdu_status
/ pdu_power_cycle / pdu_disconnect.

- McpRegistrar now registers both etterms-serial and etterms-pdu in one click
- Settings -> AI MCP shows both server paths
- publish target (PublishMcpServers) bundles both MCP servers
- bump version to 0.3.0, update About changelog, ARCHITECTURE.md, CLAUDE.md

src/ETTerms.PduMcp/PduRegistry.cs

@@ -0,0 +1,31 @@
+using System.Collections.Concurrent;
+
+namespace ETTerms.PduMcp;
+
+/// <summary>
+/// Process-wide registry of connected PDUs, keyed by IP. The MCP server is long-lived,
+/// so connections established by <c>pdu_connect</c> persist across tool calls.
+/// </summary>
+public sealed class PduRegistry
+{
+    public static readonly PduRegistry Instance = new();
+
+    /// <summary>iPoMan II/III outlet count.</summary>
+    public const int PortCount = 12;
+
+    private readonly ConcurrentDictionary<string, PduController> _pdus = new(StringComparer.OrdinalIgnoreCase);
+
+    public PduController GetOrAdd(string ip) =>
+        _pdus.GetOrAdd(ip.Trim(), static k => new PduController(k));
+
+    public bool TryGet(string ip, out PduController pdu) =>
+        _pdus.TryGetValue(ip.Trim(), out pdu!);
+
+    public bool Remove(string ip)
+    {
+        if (_pdus.TryRemove(ip.Trim(), out var pdu)) { pdu.Dispose(); return true; }
+        return false;
+    }
+
+    public IReadOnlyCollection<string> ConnectedIps => _pdus.Keys.ToArray();
+}