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.

sandboxes/orca_inset_plugin_any.py

@@ -19,8 +19,8 @@ geometry. At Step.posSlice the split slice loop runs make_perimeters() right aft
 the hook, so the change cascades into perimeters, infill and the final G-code
 -- the toolpath preview shrinks.
 
-Unlike the old axis-aligned demo, ExPolygon.offset() is a correct inward offset
-for any contour (it is Clipper under the hood), and it naturally handles holes.
+ExPolygon.offset() is a correct inward offset for any contour (it is Clipper
+under the hood), and it naturally handles holes.
 A surface may split into several islands or vanish when shrunk; both are handled.
 
 No numpy required: the whole edit is expressed with the host geometry classes.