Add script-driven UI test harness for internal builds

[Randalphwa]

Summary

Add a debug-only script-driven harness that can drive the running editor's UI from a command file, for automated UI testing under a debugger.

Motivation

An external agent/debugger needs to exercise editor UI actions (menu commands, context-menu commands, selection, undo/redo) against a live wxUiEditor instance, observe results, and answer modal dialogs — without a human at the keyboard. Synthetic wxKeyEvents bypass wxWidgets accelerator processing, so the harness drives real menu/undo code paths directly.

Changes

Core: UI harness (src/internal/ui_harness.cpp/.h, compiled only in INTERNAL_BLD_TESTING builds)

• ui_harness::Start(script, log) appends commands to a script file; a 100 ms timer reads it and executes at most one command per tick, so a breakpoint in ExecuteOne() yields exactly one debugger stop per command.
• Verbs: menu (by id or case-insensitive label / Menu/Sub/Item path, with ambiguity reporting; optional checked/unchecked state), key (resolved from the live menu bar's accelerator text rather than a synthetic key event), navmenu (builds a NavPopupMenu, runs UpdateUI so per-item preconditions apply, invokes by id or label), select, undo, redo, func (registry extension point for non-menu actions), dialogs (scripted modal answers), dump tree|menubar|navmenu|undo|selection, marker, quit.
• Node addressing via canonical /0/1/2 index paths or declared names (names are not unique, so canonical paths are emitted); . or empty resolves to the current selection.
• UTF-8 log written with flush-per-line so an agent can read results while the process is paused at a breakpoint; falls back to stderr if the log file can't open. Dialog interceptions are logged via an observer; commented lines and CRLF tolerated.

Enabling changes

• src/panels/navpopupmenu.{h,cpp}: new NavPopupMenu::InvokeCommand(int) — resolves the item by walking the item list recursively (wxMenu::FindItem does not reliably descend into submenus), rejects separators/disabled items, and dispatches synchronously via ProcessEvent so effects are immediately observable.
• src/utils/cli_ui_guard.{h,cpp}: debug-only SetScriptedDialogAnswer() (a modal-hook Enter() override returns the scripted answer instead of showing the dialog) and SetDialogObserver() to record each intercepted dialog's message and answer; wxID_NONE restores previous behavior.
• src/mainapp.cpp: hidden --dbg-ui-script / --dbg-ui-log options (log defaults to <script>.log); ui_harness::Start() is called after the main frame is shown.