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/slic3r/GUI/Tab.cpp

@@ -1794,7 +1794,7 @@ void Tab::on_value_change(const std::string& opt_key, const boost::any& value)
     // always carries resolved "name;uuid;capability" references that full_config() and save_to_json()
     // then pass downstream as-is -- no separate rebuild anywhere else.
     if (const ConfigOptionDef* opt_def = m_config->def()->get(opt_key);
-        opt_def && opt_def->plugin_type != ConfigOptionDef::PluginType::None)
+        opt_def && opt_def->is_plugin_backed())
         m_config->update_plugin_manifest();
 
     if (opt_key == "gcode_flavor" && m_type == Preset::TYPE_PRINTER) {
@@ -3080,12 +3080,6 @@ void TabPrint::build()
         option.opt.height = 15;
         optgroup->append_single_option_line(option, "others_settings_post_processing_scripts");
 
-        optgroup = page->new_optgroup(L("Post-processing Plugin"), L"param_gcode", 0);
-        optgroup->hide_labels();
-        option = optgroup->get_option("post_process_plugin");
-        option.opt.full_width = true;
-        optgroup->append_single_option_line(option, "others_settings_plugin_picker");
-
         optgroup = page->new_optgroup(L("Slicing Pipeline Plugin"), L"param_gcode", 0);
         optgroup->hide_labels();
         option = optgroup->get_option("slicing_pipeline_plugin");