docs(plugin): make comments self-contained; drop design-doc and dead-code references

Review the slicing-pipeline plugin comments for context a reader of the source
alone cannot follow, and rewrite them to stand on their own:

- drop pointers to uncommitted design/plan material ("§3.6 (Twistify design)",
  "the brief's note", "Fix 4(a)/4(b)")
- fix dangling references to code this branch removed: the retired set_slices()
  and view mutators, the former G-code post-processing capability/trampoline,
  the "Post-processing" capability family, the pre-refactor array helper
- drop "v1"/"in v1" phase labels, keeping the behavior they described
- correct stale cross-references: Twistify.py -> the real sample path;
  test_plugin_host_api.cpp:32-40 -> import_orca_module in python_test_support.hpp;
  "the binding"/"graphs above" -> the named source

Comment/string-only; no code behavior change.

src/libslic3r/Print.hpp

@@ -103,7 +103,7 @@ enum class SlicingPipelineStepPlugin {
     posSlice, posPerimeters, posEstimateCurledExtrusions, posPrepareInfill, posInfill, posIroning, posContouring,
     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.
+    // At this step the plugin edits the exported G-code file in place; see SlicingPipelinePluginCapability for the full contract.
     psGCodePostProcess
 };