revert: restore audit-log kill switch — async redesign didn't help

User pulled d9e32e5 (async-writer audit log + re-enabled diagnostics
sidebar) and still sees blank pages. The synchronous-write theory
from the previous round was at most a partial explanation; something
ELSE in the audit-log code path is also taking the page render down
on the user's machine.

Restore the kill switch so the user has a working app while we
diagnose:

- ``src/audit.py``: ``_DISABLED = True`` re-introduced at module
  top, each of ``log_event`` / ``log_session_start`` /
  ``log_page_open`` / ``flush_audit_log`` early-returns. The async
  writer thread is never started.
- ``hide_streamlit_chrome``: ``_render_diagnostics_sidebar()`` call
  re-gated behind ``if False:``.

The async writer code stays in place — easier to flip the flag back
when we identify the real cause than to rewrite a third time. The
shutdown-flush call in ``shutdown_app`` also stays; it early-returns
on the kill switch and is harmless.

Diagnostic plan for the next session: ask the user for the launcher
terminal output (the new stderr "DataTools audit: writes failing..."
message would tell us if the writer thread DID start and DID fail),
and whether ``~/.datatools/logs/`` is being created at all.

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

src/audit.py

@@ -55,6 +55,15 @@ _LOG_PATH: Path | None = None
 _SESSION_ID: str | None = None
 _SESSION_STARTED: bool = False
 
+# RESTORED kill switch — the async-writer redesign still triggers the
+# blank-pages symptom on the user's machine despite no synchronous
+# file I/O on the request path. Cause is not yet identified; keep all
+# log_* calls as no-ops while we diagnose so the GUI keeps working.
+# Diagnostic plan: ask the user for the launcher terminal output AND
+# whether anything appears in ``~/.datatools/logs/`` at all — that
+# bisects "writer thread starts" from "writer thread can't write."
+_DISABLED: bool = True
+
 # Bounded in-memory queue. ``deque(maxlen=N)`` drops the OLDEST entry
 # when full so the most-recent events are always the ones that
 # survive on disk — diagnostic-most-valuable at the moment of a
@@ -205,6 +214,8 @@ def log_event(
     Failures inside this function are swallowed; a broken audit log
     must never take the GUI down.
     """
+    if _DISABLED:
+        return
     try:
         try:
             ts = datetime.now(tz=timezone.utc).isoformat(timespec="milliseconds")
@@ -234,6 +245,8 @@ def log_event(
 
 def log_session_start() -> None:
     """Idempotent session-start banner with platform info."""
+    if _DISABLED:
+        return
     global _SESSION_STARTED
     with _LOCK:
         if _SESSION_STARTED:
@@ -260,6 +273,8 @@ def log_session_start() -> None:
 
 def log_page_open(slug: str) -> None:
     """Emit a deduplicated 'page open' nav event."""
+    if _DISABLED:
+        return
     try:
         try:
             import streamlit as st
@@ -298,6 +313,8 @@ def flush_audit_log(timeout_s: float = 0.5) -> None:
     stuck disk can never delay shutdown — events still in the queue
     when the timer expires are dropped.
     """
+    if _DISABLED:
+        return
     global _SHUTDOWN_REQUESTED
     deadline = time.monotonic() + max(0.0, timeout_s)
     with _QUEUE_COND: