Housekeeping ahead of the Inc 1-4 coverage work

- inventory.py --methods: report the instance methods the app calls per class,
  plus chained calls that constrain a return type. --check says which classes
  to bind; this says what to bind on them, which is what writing ~38 module
  TUs needs.
- Drop StlAPI from the inventory: StlAPI_Writer has no app call site (the only
  use was a test fixture, now on the app's own STL writer). 138 symbols / 47
  modules.
- adding-symbols.md: scope the executing-constructor ban to the BRepAlgoAPI
  booleans, which are the only classes with a deferred Set*/Build form —
  BRepMesh_IncrementalMesh, GeomAPI_*, BRepCheck_Analyzer and friends compute
  in their constructor by design and bind as stock. Replace the per-increment
  app-test guidance: backend/tests/conftest.py imports n3xd.main, so no app
  test can collect until the last module is bound. Increments gate on
  stock-recorded fixtures here; the app suite is the Inc 4 gate.
- parity_venv.sh: run the ocp suite in the swapped venv (it imports only
  OCP/n3xd_ocp, so it works throughout).
- Fix a stale macro name in occt_handle.h (ocp_new, not OCP_TRANSIENT_NEW) and
  drop the unused ocp_transient_class helper.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DfriM8XUkn7uYf5Dwe2xo6

inventory.json

@@ -212,9 +212,6 @@
       "StdPrs_BRepFont": [],
       "StdPrs_BRepTextBuilder": []
     },
-    "StlAPI": {
-      "StlAPI_Writer": []
-    },
     "TColStd": {
       "TColStd_Array1OfInteger": [],
       "TColStd_Array1OfReal": [],