fix(test): drop rmtree race in live_gui workspace creation

The session-scoped live_gui fixture deleted the shared workspace
before recreating it, which raced with the per-worker lock acquisition
and produced FileNotFoundError on .live_gui_owner.lock in xdist.
The per-run timestamped name (tests/artifacts/live_gui_workspace_<ts>/)
already provides enough isolation between pytest invocations, so the
rmtree is unnecessary. Use mkdir(exist_ok=True) only.

tests/conftest.py

@@ -478,17 +478,16 @@ def live_gui(request) -> Generator["_LiveGuiHandle", None, None]:
 
  # 1. Create the run workspace (one per pytest invocation, shared by the
  #    single owner worker that actually runs live_gui tests).
+ # Per-run isolation: each `uv run pytest` gets a fresh timestamped folder.
+ # Per-test pollution is INTENTIONAL (exposes fragility, per the
+ # workspace_path_finalize_20260609 spec).
+ # NOTE: do NOT rmtree the workspace here. With xdist, 16 workers share this
+ # path concurrently; an rmtree-by-any-worker would delete the dir between
+ # another worker's mkdir and lock acquisition, causing FileNotFoundError.
+ # The timestamp-based name provides enough isolation for normal use; if you
+ # re-run within the same second, just wait a second.
  temp_workspace = _RUN_WORKSPACE
- if temp_workspace.exists():
-  for _ in range(5):
-   try:
-    shutil.rmtree(temp_workspace)
-    break
-   except OSError:
-    time.sleep(0.5)
- # Create the workspace directory before writing files
  temp_workspace.mkdir(parents=True, exist_ok=True)
- 
  # Create minimal project files to avoid cluttering root
  (temp_workspace / "manual_slop.toml").write_text("[project]\nname = 'TestProject'\n\n[conductor]\ndir = 'conductor'\n", encoding="utf-8")
  (temp_workspace / "conductor" / "tracks").mkdir(parents=True, exist_ok=True)