feat(plugin)!: merge G-code post-processing into the slicing pipeline as psGCodePostProcess

G-code post-processing is now a step of the slicing-pipeline plugin rather than a
separate capability type. One capability class can transform slices at the geometry
seams AND edit the final G-code, behind a single picker/option.

- Add SlicingPipelineStepPlugin::psGCodePostProcess (bound as
  orca.slicing.Step.psGCodePostProcess). Unlike the geometry steps it fires from the
  GUI export path in PostProcessor.cpp, not from Print::process(): ctx.print/ctx.object
  are None and the plugin edits the file at ctx.gcode_path in place. It may run more
  than once per slice (file export and/or upload) and its output is not shown in the
  preview.
- Extend SlicingPipelineContext with gcode_path/host/output_name and a C++-only
  full_config; config_value() falls back to it when there is no live Print.
- PostProcessor.cpp dispatches SlicingPipelinePluginCapability at psGCodePostProcess,
  driven by the existing slicing_pipeline_plugin option.
- The exported G-code lives outside data_dir(), so the plugin audit sandbox would
  block the write; the trampoline's audit setup grants ctx.gcode_path's folder as a
  scoped allowed root, gated on a non-empty gcode_path so the geometry-step hooks gain
  no extra filesystem access.

BREAKING CHANGE: the separate G-code post-processing capability type is removed.
- orca.gcode.GCodePluginCapabilityBase and orca.PluginType.PostProcessing are gone;
  post-processing plugins migrate to orca.slicing.SlicingPipelineCapabilityBase +
  Step.psGCodePostProcess (and gain ctx.params / ctx.config_value()).
- The post_process_plugin config option is removed; use slicing_pipeline_plugin.
  Presets carrying the old key degrade to the standard unknown-key warning.
- Manifest type = "post-processing" now maps to Unknown (advisory only; the loader
  dispatches on the C++ get_type()).

Also repairs two latent build breaks the branch carried: stale Step enum value usages
in test_slicing_pipeline_hook.cpp and a reference to the removed
ConfigOptionDef::PluginType::None in Tab::on_value_change (now is_plugin_backed()).
Adds the orca_gcode_stamp sample plugin and a psGCodePostProcess binding test.

src/libslic3r/Config.hpp

@@ -2444,9 +2444,9 @@ public:
     // "serialized" - vector valued option is entered in a single edit field. Values are separated by a semicolon.
     // "show_value" - even if enum_values / enum_labels are set, still display the value, not the enum label.
     std::string                         gui_flags;
-    // Capability type of a plugin-backed option, e.g. "post-processing" / "slicing-pipeline" /
-    // "printer-connection" (empty for ordinary options). GUIType::plugin_picker filters the plugin
-    // list by it, and it resolves the option's "plugins" manifest reference; see is_plugin_backed().
+    // Capability type of a plugin-backed option, e.g. "slicing-pipeline" / "printer-connection"
+    // (empty for ordinary options). GUIType::plugin_picker filters the plugin list by it, and it
+    // resolves the option's "plugins" manifest reference; see is_plugin_backed().
     std::string                         plugin_type;
     // Whether this option holds plugin capability name(s) that feed the "plugins" manifest -- true
     // iff it declares a plugin_type. Setting plugin_type is the only step needed to add one.

src/libslic3r/Preset.cpp

@@ -1192,7 +1192,6 @@ static std::vector<std::string> s_Preset_print_options{
     "min_feature_size",
     "min_bead_width",
     "post_process",
-    "post_process_plugin",
     "slicing_pipeline_plugin",
     "plugins",
     "process_change_extrusion_role_gcode",

src/libslic3r/Print.cpp

@@ -125,8 +125,8 @@ bool Print::invalidate_state_by_config_options(const ConfigOptionResolver & /* n
         "printing_by_object_gcode",
         "filament_end_gcode",
         "post_process",
-        "post_process_plugin",
-        // "plugins" is the manifest backing post_process_plugin; like it, it only affects G-code export.
+        // "plugins" is the derived manifest backing the plugin-picker options; on its own it only
+        // affects G-code export. The specific option (e.g. slicing_pipeline_plugin) drives any re-slice.
         "plugins",
         "extruder_clearance_height_to_rod",
         "extruder_clearance_height_to_lid",

src/libslic3r/Print.hpp

@@ -101,7 +101,10 @@ enum PrintObjectStep {
 
 enum class SlicingPipelineStepPlugin {
     posSlice, posPerimeters, posEstimateCurledExtrusions, posPrepareInfill, posInfill, posIroning, posContouring,
-    posSupportMaterial, posDetectOverhangsForLift, posSimplifyPath, psWipeTower, psSkirtBrim
+    posSupportMaterial, posDetectOverhangsForLift, posSimplifyPath, psWipeTower, psSkirtBrim,
+    // Fires from the GUI G-code export/post-process seam (PostProcessor.cpp), NOT from Print::process().
+    // At this step the plugin edits the exported G-code file in place; see the binding for the full contract.
+    psGCodePostProcess
 };
 
 // A PrintRegion object represents a group of volumes to print

src/libslic3r/PrintConfig.cpp

@@ -5114,20 +5114,10 @@ void PrintConfigDef::init_fff_params()
     def->mode = comDevelop;
     def->set_default_value(new ConfigOptionStrings());
 
-    def = this->add("post_process_plugin", coStrings);
-    def->label = L("Post-processing Plugin");
-    def->tooltip = L("Select a Python plugin to process the output G-code. "
-                   "Plugins are loaded from the orca_plugins directory in your data folder. "
-                   "The plugin will receive the G-code file path and can modify it in place.");
-    def->gui_type = ConfigOptionDef::GUIType::plugin_picker;
-    def->plugin_type = "post-processing";
-    def->full_width = true;
-    def->mode = comAdvanced;
-    def->set_default_value(new ConfigOptionStrings());
-
     def = this->add("slicing_pipeline_plugin", coStrings);
     def->label = L("Slicing Pipeline Plugin");
-    def->tooltip = L("Python plugin(s) invoked at each slicing pipeline step to read and modify intermediate slicing data. Research/experimental.");
+    def->tooltip = L("Python plugin(s) invoked at each slicing pipeline step to read and modify intermediate slicing data, "
+                   "including a final G-code post-processing step. Research/experimental.");
     def->gui_type = ConfigOptionDef::GUIType::plugin_picker;
     def->plugin_type = "slicing-pipeline";
     def->full_width = true;

src/libslic3r/PrintConfig.hpp

@@ -1570,7 +1570,6 @@ PRINT_CONFIG_CLASS_DERIVED_DEFINE(
     ((ConfigOptionBool,               ooze_prevention))
     ((ConfigOptionString,             filename_format))
     ((ConfigOptionStrings,            post_process))
-    ((ConfigOptionStrings,            post_process_plugin))
     ((ConfigOptionStrings,            slicing_pipeline_plugin))
     ((ConfigOptionString,             printer_model))
     ((ConfigOptionFloat,              resolution))