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/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",