[P2] Add teardown for streamlit_app modules after DummyStreamlit reload tests

[stranske]

Why

tests/streamlit/test_mc_page.py:246-251 reloads streamlit_app.* modules while streamlit is stubbed; tests/baseline/test_streamlit_smoke.py:37-47 documents that monkeypatch does not restore reloaded page modules. This is a latent flake: AppTest smoke tests can render into a dummy and pass vacuously unless modules are evicted manually.

Scope

Shared pytest fixture/autouse teardown that removes streamlit_app entries from sys.modules after DummyStreamlit tests. Add an order-regression test that runs MC stub test then AppTest smoke without manual eviction.

Implementation Notes

• Implement fixture in tests/streamlit/conftest.py or tests/conftest.py.
• Reuse eviction list logic from tests/baseline/test_streamlit_smoke.py:44-47.
• Do not remove DummyStreamlit unit tests; they remain valid for fast widget logic checks.
• Coordinate with any existing tests/baseline/harness.py utilities.

Tasks

• Add autouse teardown fixture in tests/streamlit/conftest.py evicting streamlit_app* from sys.modules
• Add test_stub_reload_does_not_break_apptest in tests/baseline/test_streamlit_smoke.py that chains _load_page + _run_page
• Import shared _load_page helper or duplicate minimal stub install in the regression test only if import cycles forbid sharing

Acceptance Criteria

• pytest tests/baseline/test_streamlit_smoke.py tests/streamlit/test_mc_page.py -q passes
• new regression test fails if teardown fixture is disabled
• temporarily comment out the teardown fixture in tests/streamlit/conftest.py and confirm test_stub_reload_does_not_break_apptest fails; revert

Non-Goals

• Migrating all Streamlit unit tests to AppTest
• Fixing numpy collection errors in tests/streamlit/test_mc_plots.py

Scaffold-only completion does NOT count: a fixture that evicts modules but has no regression test proving AppTest still sees real Streamlit is a failure of this issue.