fix: detect Mercury stage per-view via session name

The Mercury app (3.2.x) is a hybrid server whose kernel pool serves both
the client-facing app view and JupyterLab, so the server-level
MERCURY_CONFIG_DIR signal cannot tell who is looking and leaks backstage
content in the app. Switch the primary stage signal to the shadow-copy
session name (__mercury__ in JPY_SESSION_NAME), keeping the env var only
as a --working-dir fallback.

- Update CX Discovery staging.py to the per-view detection model
- Ignore Mercury runtime artifacts (.mercury_sessions/, *__mercury__*)
- Document the leak in remaining masters (TEI, CTM, AI Diagnostic,
  template) and flag the staging.py retrofit as urgent

tests/nbcheck.py

@@ -70,6 +70,11 @@ def discover() -> list[str]:
         for p in root.rglob("*.ipynb"):
             if ".ipynb_checkpoints" in p.parts or ".venv" in p.parts:
                 continue
+            # Mercury app shadow copies are runtime artifacts, not masters
+            # (layout varies by mercury version: .mercury_sessions/ or a
+            # __mercury__ suffix beside the notebook).
+            if ".mercury_sessions" in p.parts or "__mercury__" in p.name:
+                continue
             if p.parent.name != "notebooks":
                 continue
             found.append(p.relative_to(REPO).as_posix())