Add MCP bridge for external tool integration

Embedded JSON-RPC server over named pipes (rcx-mcp) enabling external
tools like Claude Code to inspect and manipulate the node tree, read/write
hex data, switch sources, and trigger UI actions. Includes stdio adapter
(rcx-mcp-stdio) for stdin/stdout transport. Server is stopped by default;
user starts via File > Start MCP Server.

Also extracts MainWindow class declaration to mainwindow.h and improves
type selector popup Esc button styling.

src/mcp/mcp_bridge.h

@@ -0,0 +1,67 @@
+#pragma once
+#include "mainwindow.h"
+#include <QObject>
+#include <QLocalServer>
+#include <QLocalSocket>
+#include <QJsonObject>
+#include <QJsonArray>
+#include <QJsonDocument>
+#include <QByteArray>
+
+namespace rcx {
+
+class McpBridge : public QObject {
+    Q_OBJECT
+public:
+    explicit McpBridge(MainWindow* mainWindow, QObject* parent = nullptr);
+    ~McpBridge() override;
+
+    void start();
+    void stop();
+    bool isRunning() const { return m_server != nullptr; }
+
+    // Call from controller refresh / data change to notify MCP clients
+    void notifyTreeChanged();
+    void notifyDataChanged();
+
+private:
+    MainWindow*    m_mainWindow;
+    QLocalServer*  m_server    = nullptr;
+    QLocalSocket*  m_client    = nullptr;  // single client for v1
+    QByteArray     m_readBuffer;
+    bool           m_initialized = false;
+
+    // JSON-RPC plumbing
+    void onNewConnection();
+    void onReadyRead();
+    void onDisconnected();
+    void processLine(const QByteArray& line);
+    void sendJson(const QJsonObject& obj);
+    QJsonObject okReply(const QJsonValue& id, const QJsonObject& result);
+    QJsonObject errReply(const QJsonValue& id, int code, const QString& msg);
+    void sendNotification(const QString& method, const QJsonObject& params = {});
+
+    // MCP method handlers
+    QJsonObject handleInitialize(const QJsonValue& id, const QJsonObject& params);
+    QJsonObject handleToolsList(const QJsonValue& id);
+    QJsonObject handleToolsCall(const QJsonValue& id, const QJsonObject& params);
+
+    // Tool implementations
+    QJsonObject toolProjectState(const QJsonObject& args);
+    QJsonObject toolTreeApply(const QJsonObject& args);
+    QJsonObject toolSourceSwitch(const QJsonObject& args);
+    QJsonObject toolHexRead(const QJsonObject& args);
+    QJsonObject toolHexWrite(const QJsonObject& args);
+    QJsonObject toolStatusSet(const QJsonObject& args);
+    QJsonObject toolUiAction(const QJsonObject& args);
+
+    // Helpers
+    QJsonObject makeTextResult(const QString& text, bool isError = false);
+    QString resolvePlaceholder(const QString& ref,
+                               const QHash<QString, uint64_t>& placeholderMap);
+
+    // Smart tab resolution: tabIndex arg → activeTab → first tab → auto-create
+    MainWindow::TabState* resolveTab(const QJsonObject& args);
+};
+
+} // namespace rcx