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

assessments/CX_Discovery_Workshop/tests/test_staging.py

@@ -1,14 +1,47 @@
-"""Stage/backstage detection — Mercury kernels carry MERCURY_CONFIG_DIR."""
+"""Stage/backstage detection.
+
+Two signals mark the Mercury app view (see staging.py): the app's shadow
+session name (``__mercury__`` in ``JPY_SESSION_NAME`` — per-view, primary)
+and the ``MERCURY_CONFIG_DIR`` env var (server-level fallback, only set by
+``mercury --working-dir``). Either one means "a client may be looking".
+"""
+
+import pytest
 
 from discoverylib import staging
 
 
-def test_backstage_prints_only_off_stage(monkeypatch, capsys):
+@pytest.fixture(autouse=True)
+def clean_stage_env(monkeypatch):
     monkeypatch.delenv("MERCURY_CONFIG_DIR", raising=False)
+    monkeypatch.delenv("JPY_SESSION_NAME", raising=False)
+
+
+def test_backstage_by_default(capsys):
     assert not staging.on_stage()
     staging.backstage("visible")
     assert capsys.readouterr().out == "visible\n"
 
+
+def test_plain_session_name_is_backstage(monkeypatch, capsys):
+    # A JupyterLab or nbconvert session: the real notebook path, no marker.
+    monkeypatch.setenv("JPY_SESSION_NAME", "notebooks/cx_discovery.ipynb")
+    assert not staging.on_stage()
+    staging.backstage("visible")
+    assert capsys.readouterr().out == "visible\n"
+
+
+def test_mercury_shadow_session_is_stage(monkeypatch, capsys):
+    # The Mercury app runs against a shadow copy: <stem>__mercury__<id>.ipynb
+    monkeypatch.setenv(
+        "JPY_SESSION_NAME", "notebooks/cx_discovery__mercury__545e5520.ipynb"
+    )
+    assert staging.on_stage()
+    staging.backstage("hidden")
+    assert capsys.readouterr().out == ""
+
+
+def test_config_dir_fallback_is_stage(monkeypatch, capsys):
     monkeypatch.setenv("MERCURY_CONFIG_DIR", "/tmp/app")
     assert staging.on_stage()
     staging.backstage("hidden")
@@ -18,11 +51,12 @@ def test_backstage_prints_only_off_stage(monkeypatch, capsys):
 def test_backstage_md_renders_only_off_stage(monkeypatch, capsys):
     # Off stage it must emit SOMETHING (rich markdown under a kernel;
     # IPython's display degrades to print under plain pytest) …
-    monkeypatch.delenv("MERCURY_CONFIG_DIR", raising=False)
     staging.backstage_md("**visible**")
     assert capsys.readouterr().out != ""
 
-    # … and on stage, nothing at all.
-    monkeypatch.setenv("MERCURY_CONFIG_DIR", "/tmp/app")
+    # … and on stage — either signal — nothing at all.
+    monkeypatch.setenv(
+        "JPY_SESSION_NAME", "notebooks/cx_discovery__mercury__ab12cd34.ipynb"
+    )
     staging.backstage_md("**hidden**")
     assert capsys.readouterr().out == ""