Read IMEX geometry defaults from print_config_def instead of literals

Five sites carried a hardcoded fallback for imex_tools_per_gantry that had
to match the value registered in print_config_def, with nothing enforcing
the agreement, and several explanatory comments miscounted the sites they
described or cited stale line numbers.

Add imex_cfg_int/_float/_bool/_enum<T> to IMEXHelpers, which return the
value registered for the key when it is absent from the config, so the
registration is the single source and there is nothing left to keep in
sync. Route every read of the IMEX geometry keys through them: 32 call
sites across IMEXZones, PartPlate, GCodeViewer and Tab. The only direct
lookup left is the bail in PartPlate::imex_multicolor_block_reason, which
must not default because it reports a routing conflict and a defaulted
grid would produce a false warning.

imex_cfg_enum uses dynamic_cast on both halves rather than the type()
comparison the others use: every ConfigOptionEnum<T> reports coEnum, so a
type() check cannot tell one enum type from another and would cast a
ConfigOptionEnum<OtherEnum> to the requested T. The ConfigOptionPercent :
ConfigOptionFloat inheritance that rules dynamic_cast out for the float
accessor has no analogue for enums.

Correct the comments that prompted this: the cache-key input list in
PartPlate named five inputs for a nine-part key, the ImexMarkerKey note
in GCodeViewer called imex_tool_layout an input only the preview reads
when the plate keys it too, and four file:line citations pointed at the
wrong lines. Values are unchanged at every converted site; cache key
strings keep their existing representation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/slic3r/GUI/Tab.cpp

@@ -5489,6 +5489,18 @@ PageShp TabPrinter::build_kinematics_page()
  * but "Motion ability" and "Single extruder MM setup" too
  * (These pages can changes according to the another values of a current preset)
  * */
+// Grid shape for the IMEX modes editor, for the three sites that build or resize the control:
+// build_unregular_pages(), reload_config() and update_fff(). They must read these keys the way
+// compute_imex_zone_layout() does, so this goes through the shared accessors rather than
+// opt_int(), which does not throw on an absent key -- it dereferences the null that
+// option<ConfigOptionInt>() returns (Config.hpp:2991), where every other reader falls back.
+static void imex_grid_shape(const DynamicPrintConfig* cfg, int& n_cols, int& n_rows, int& layout)
+{
+    n_cols = std::max(1, imex_cfg_int(*cfg, "imex_tools_per_gantry"));
+    n_rows = std::max(1, imex_cfg_int(*cfg, "imex_gantry_count"));
+    layout = IMEXModesCtrl::parse_layout(imex_cfg_enum<ImexToolLayout>(*cfg, "imex_tool_layout"));
+}
+
 void TabPrinter::build_unregular_pages(bool from_initial_build/* = false*/)
 {
     size_t		n_before_extruders = 2;			//	Count of pages before Extruder pages
@@ -5682,11 +5694,9 @@ if (is_marlin_flavor)
             auto line = Line{ L("Modes"), L("") };
             line.full_width = 1;
             line.widget = [this](wxWindow* parent) -> wxSizer* {
-                int n_cols = m_config->opt_int("imex_tools_per_gantry");
-                int n_rows = m_config->opt_int("imex_gantry_count");
-                ImexToolLayout layout = ImexToolLayout::FrontLeft;
-                if (auto* o = m_config->option<ConfigOptionEnum<ImexToolLayout>>("imex_tool_layout")) layout = o->value;
-                m_imex_modes_ctrl = new IMEXModesCtrl(parent, n_cols, n_rows, IMEXModesCtrl::parse_layout(layout));
+                int n_cols = 0, n_rows = 0, layout = 0;
+                imex_grid_shape(m_config, n_cols, n_rows, layout);
+                m_imex_modes_ctrl = new IMEXModesCtrl(parent, n_cols, n_rows, layout);
                 // Lazy lookup pointing at the *saved* state of the currently-selected
                 // preset (not the parent). Per-row reset means "discard in-session
                 // edits to this row" — matches the page-level reset semantic and works
@@ -6052,11 +6062,9 @@ void TabPrinter::reload_config()
         // standard Choice fields; only the modes grid still needs explicit re-sync
         // because it spans three options at once and isn't a Field.
         if (m_imex_modes_ctrl) {
-            int n_cols = m_config->opt_int("imex_tools_per_gantry");
-            int n_rows = m_config->opt_int("imex_gantry_count");
-            ImexToolLayout layout = ImexToolLayout::FrontLeft;
-            if (auto* o = m_config->option<ConfigOptionEnum<ImexToolLayout>>("imex_tool_layout")) layout = o->value;
-            m_imex_modes_ctrl->set_grid_size(n_cols, n_rows, IMEXModesCtrl::parse_layout(layout));
+            int n_cols = 0, n_rows = 0, layout = 0;
+            imex_grid_shape(m_config, n_cols, n_rows, layout);
+            m_imex_modes_ctrl->set_grid_size(n_cols, n_rows, layout);
             m_imex_modes_ctrl->load_from_config(*m_config);
         }
     }
@@ -6307,10 +6315,10 @@ void TabPrinter::toggle_options()
         if (is_imex) {
             if (Field* layout_field = get_field("imex_tool_layout"); layout_field) {
                 if (auto* choice = dynamic_cast<Choice*>(layout_field); choice) {
-                    const int gantry_count = m_config->opt_int("imex_gantry_count");
-                    int current_val = 0;
-                    if (auto* o = m_config->option<ConfigOptionEnum<ImexToolLayout>>("imex_tool_layout"))
-                        current_val = static_cast<int>(o->value);
+                    const int gantry_count = std::max(1, imex_cfg_int(*m_config, "imex_gantry_count"));
+                    // Not const: the rear-* -> front-* normalization below reassigns it.
+                    int current_val =
+                        static_cast<int>(imex_cfg_enum<ImexToolLayout>(*m_config, "imex_tool_layout"));
 
                     // Normalize rear-* → front-* when collapsing to 1 gantry.
                     if (gantry_count == 1 && (current_val == static_cast<int>(ImexToolLayout::RearLeft)
@@ -6583,11 +6591,9 @@ void TabPrinter::update_fff()
     // refreshes the rows because that path actually changes the config, which
     // matches_config() then detects.
     if (m_imex_modes_ctrl) {
-        int n_cols = m_config->opt_int("imex_tools_per_gantry");
-        int n_rows = m_config->opt_int("imex_gantry_count");
-        ImexToolLayout layout = ImexToolLayout::FrontLeft;
-        if (auto* o = m_config->option<ConfigOptionEnum<ImexToolLayout>>("imex_tool_layout")) layout = o->value;
-        m_imex_modes_ctrl->set_grid_size(n_cols, n_rows, IMEXModesCtrl::parse_layout(layout));
+        int n_cols = 0, n_rows = 0, layout = 0;
+        imex_grid_shape(m_config, n_cols, n_rows, layout);
+        m_imex_modes_ctrl->set_grid_size(n_cols, n_rows, layout);
         if (!m_imex_modes_ctrl->matches_config(*m_config))
             m_imex_modes_ctrl->load_from_config(*m_config);
     }