fix(rag): stop live_gui tests from polluting session-scoped subprocess

Per Tier 1 investigation
(docs/reports/INVESTIGATION_rag_phase4_final_verify_20260627.md),
two live_gui tests were leaking temp/relative paths into the shared
subprocess's ui_files_base_dir, which survived across @clean_baseline
tests and caused RAGEngine.index_file to silently no-op on a dead
base_dir.

Three fixes:

1. tests/test_rag_visual_sim.py: stop using tempfile.mkdtemp() (which
   defaults to C:\Users\Ed\AppData\Local\Temp\tmpXXXX) and instead use
   tempfile.mkdtemp(dir="tests/artifacts", ...). Also restore
   files_base_dir and rag_enabled in finally so the next live_gui test
   in the session doesn't inherit the dead path.

2. tests/test_visual_sim_mma_v2.py: stop changing files_base_dir to
   'tests/artifacts/temp_workspace' and stop clicking btn_project_save
   (which persisted the path to manual_slop.toml). The MMA lifecycle
   does not depend on a specific files_base_dir.

3. src/app_controller.py _handle_reset_session: defensive fix that
   resets ui_files_base_dir from the default project's base_dir. This
   makes reset_session() robust to any future polluter (not just the
   two known ones). Without this, a test that sets files_base_dir via
   set_value leaves a dead path in the session-scoped subprocess even
   after reset_session().

Verified: tests/test_rag_visual_sim.py passes 2/2 after the fix.

src/app_controller.py

@@ -3893,6 +3893,14 @@ class AppController:
   self.ui_ai_input = ""
   self.ui_manual_approve = False
   self.ui_auto_add_history = False
+  # Reset ui_files_base_dir to a sane default so a prior live_gui test
+  # that set a temp/relative path via set_value('files_base_dir', ...)
+  # does not pollute the session-scoped subprocess with a dead path.
+  # See tests/test_rag_visual_sim.py and tests/test_visual_sim_mma_v2.py
+  # for the original polluters. Per Tier 1 investigation:
+  # docs/reports/INVESTIGATION_rag_phase4_final_verify_20260627.md
+  self.ui_files_base_dir = self.project.get("files", {}).get("base_dir", ".")
+  self.ui_shots_base_dir = self.project.get("screenshots", {}).get("base_dir", ".")
   self.active_track = None
   self.active_tier = None
   self.mma_status = 'idle'