fix(audit): break audit_log_path/_session_id deadlock

Pre-existing latent bug since d9e32e5: ``audit_log_path()`` acquires
the non-reentrant ``_LOCK`` and, while holding it, calls
``_session_id()`` which also takes ``_LOCK``. On a clean module state
(both ``_LOG_PATH`` and ``_SESSION_ID`` unset) the first caller
deadlocks.

``log_session_start`` triggers it in practice — it's the first GUI
call after import and the ``log_file=str(audit_log_path())`` arg is
evaluated before any ``log_event`` has had a chance to lazy-init the
session id. Strong candidate contributor to the blank-pages symptom
the kill switch was put back to mask: the writer thread (and any
producer reaching ``audit_log_path``) would freeze forever, and
Ctrl+C would not free the GIL — matches the launcher-can't-be-killed
behaviour reported in 1caedbb.

Fix: resolve the session id BEFORE acquiring ``_LOCK`` in
``audit_log_path``. ``_session_id`` already double-checks under its
own lock, so the call is safe and self-synchronising.

Test fixture in ``tests/test_audit.py`` now bypasses the kill switch
via ``monkeypatch.setattr(audit, "_DISABLED", False)`` — env vars are
captured at import time and ``monkeypatch.setenv`` won't reach the
module-level flag. With the fix in place, all 6 tests pass in 0.15s;
without it, ``test_session_start_renders`` (and any test exercising
the log_session_start path) hangs indefinitely.

Kill switch behaviour is unchanged in production (`_DISABLED = True`
in the shipped module); this is purely a correctness fix for the
code path that gets exercised when the switch is off.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_audit.py

@@ -26,9 +26,14 @@ import pytest
 @pytest.fixture
 def isolated_audit(monkeypatch, tmp_path):
     """Redirect audit writes into ``tmp_path`` and reset module state
-    so each test starts fresh."""
+    so each test starts fresh.
+
+    The kill switch is bypassed for the duration of the test by
+    patching the module-level constant directly — these tests need
+    the real producer path to run."""
     monkeypatch.setenv("DATATOOLS_AUDIT_DIR", str(tmp_path))
     from src import audit
+    monkeypatch.setattr(audit, "_DISABLED", False)
     audit.reset_for_tests()
     yield audit
     # Best-effort cleanup so a runaway writer thread doesn't keep
@@ -115,6 +120,7 @@ class TestUnwritableTargetDoesntCrash:
         not_a_dir.write_text("hi")
         monkeypatch.setenv("DATATOOLS_AUDIT_DIR", str(not_a_dir))
         from src import audit
+        monkeypatch.setattr(audit, "_DISABLED", False)
         audit.reset_for_tests()
         try:
             start = time.perf_counter()