docs(testing): document _LiveGuiHandle + live_gui_workspace + clean_baseline marker

docs/guide_testing.md

@@ -161,6 +161,55 @@ def test_my_thing(live_gui):
 
 ---
 
+## Per-test Subprocess Resilience (2026-06-09)
+
+Added in `test_infrastructure_hardening_20260609` track. These three mechanisms address the "subprocess state pollution" and "controller state pollution" failure modes that caused batch regressions.
+
+### `_LiveGuiHandle` class (tests/conftest.py:393)
+
+The `live_gui` fixture yields a `_LiveGuiHandle` instead of a `(process, gui_script)` tuple. The handle exposes:
+
+| Attribute/Method | Purpose |
+|---|---|
+| `process` | The `subprocess.Popen` for the sloppy.py subprocess |
+| `gui_script` | Absolute path to sloppy.py |
+| `workspace` | Absolute path to the subprocess's working directory (pytest tmp dir) |
+| `is_alive()` | True if the subprocess is running |
+| `ensure_alive()` | No-op stub — increments `respawn_count` if dead, does not respawn (deferred) |
+| `respawn_count` | Number of times the subprocess was found dead |
+
+**Backward compat:** The handle is iterable as `(process, gui_script)`, so existing `proc, _ = live_gui` patterns still work.
+
+### `live_gui_workspace` fixture (tests/conftest.py:657)
+
+Yields the absolute path to the live_gui subprocess's workspace (a `tmp_path_factory.mktemp("live_gui_workspace")` directory in pytest's tmp dir). Tests that need to create files in the workspace should request this fixture instead of hardcoding `Path("tests/artifacts/live_gui_workspace")`.
+
+```python
+def test_rag_setup(live_gui, live_gui_workspace):
+    test_file = live_gui_workspace / "my_input.txt"
+    test_file.write_text("hello")
+    # ... configure RAG, index, query
+```
+
+### `_check_live_gui_health` autouse fixture (tests/conftest.py:650)
+
+Runs before every test that uses `live_gui`. Calls `handle.ensure_alive()` to detect subprocess death between tests. If the subprocess died, the counter increments (but the subprocess is not respawned — see `ensure_alive` above).
+
+### `clean_baseline` marker
+
+Opt-in marker for tests that need a fresh controller state. Tests marked with `@pytest.mark.clean_baseline` get `/api/reset_session` called before they start, ensuring no pollution from prior tests.
+
+```python
+@pytest.mark.clean_baseline
+def test_rag_final_verify(live_gui):
+    # ai_input is guaranteed empty, controller is in a known state
+    ...
+```
+
+Use this for tests that are sensitive to controller state pollution from prior tests in the same session. The `test_rag_phase4_final_verify` test is marked this way because the 4 sims in `test_extended_sims.py` mutate controller state (provider, model, etc.) that would otherwise pollute the RAG test.
+
+---
+
 ## Test Categories
 
 ### 1. Unit Tests (no fixtures, fast)