WIP: the value field stops being a window — renders in-canvas, does not yet take keys

The decision (Tommaso's, put to him with the trade-offs): the field stops being a separate
top-level window, because whether such a window may receive typing is the window manager's
call and not ours. openbox grants it, mutter on his desktop refuses, and seven previous
workarounds fought that — one of them causing a macOS regression, and the test harness ending
up clicking the field before typing, which is a workaround no user can be asked to perform and
is exactly the "label value not editable" report.

DONE and proved on the rig: SketchInlineEditor is no longer a wxFrame + wxTextCtrl. It is state
plus an ImGui overlay drawn by DesignSketchTool::render(), at the same screen anchor, in the same
vocabulary as the dimension labels next to it (draw_dim_label is already an ImGui window). The
field opens where it should — [UX] open title=Length prefill=158.74 from the running app.

NOT DONE: typing does not reach it. ImGui is fed from GLCanvas3D's own key handler, so the keys
have to arrive at the canvas; giving the canvas wx focus when the field opens was not enough.
The remaining question is where a keystroke goes between DesignPanel's wxEVT_CHAR_HOOK and
GLCanvas3D::on_char in the Design tab, and whether the canvas repaints often enough for ImGui to
advance its input state. That is attempt three on this specific point, so it goes to a second
opinion rather than a third guess.

Also here: scripts/CAD/check-gui-click-edit.py, the ladder Tommaso asked for. It types into the
field WITHOUT clicking it first — the click is what check-gui-sketching.py's focus_field() does
and why that suite can never see this defect — and fails when the prefill is what gets committed.
It currently fails, correctly, on the above.

Not on cad-mainline: the deployed binary must stay the last good build until typing works.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011FbJKJAJxxkhDTs9XdZzKA

src/slic3r/GUI/CAD/DesignSketchTool.cpp

@@ -2,6 +2,7 @@
 #include "slic3r/GUI/GLCanvas3D.hpp"
 #include "slic3r/GUI/GUI_App.hpp"
 #include "slic3r/GUI/ImGuiWrapper.hpp"
+#include "slic3r/GUI/CAD/SketchInlineEditor.hpp"
 #include "slic3r/GUI/Plater.hpp"
 
 #include <imgui/imgui.h>
@@ -8479,6 +8480,11 @@ void DesignSketchTool::render(GLCanvas3D& canvas)
     m_dim_label_seq = 0;
     m_render_scale  = canvas.get_scale();
     emit_step_hint();   // before the early returns: an armed tool on an empty sketch still guides
+    // The value field, BEFORE every early return below. It can be up in Constrain mode on a
+    // committed feature and on an empty sketch, and a field that is not drawn is a field that is
+    // not there — there is no window to fall back to any more.
+    if (inline_editor != nullptr)
+        inline_editor->render(*wxGetApp().imgui(), m_render_scale);
     (void)canvas;
     if (!has_display()) {
         if (on_readout) on_readout(std::string());   // nothing to show -> hide HUD