Restart timing: bbctrl.Trace, /api/diag/timing, UI marks

Add a lightweight, self-contained phase tracer for measuring end-to-end
bbctrl restart and Pi boot time. Disabled by setting BBCTRL_TRACE=0.

- src/py/bbctrl/Trace.py: monotonic-anchored event log + sd_notify helper.
- bbctrl/__init__.py: marks for imports, args parsed, ioloop, web init,
  listen, and an sd_notify READY=1 once HTTP is bound.
- bbctrl/Ctrl.py: spans around each subsystem (avr, i2c, lcd, mach,
  preplanner, jog, pwr, hooks, aux, mach.connect).
- bbctrl/Comm.py: avr.firmware_rebooted mark.
- bbctrl/Web.py: TimingHandler (GET /api/diag/timing) and
  UITimingHandler (PUT /api/diag/timing/ui), plus a ws.first_open mark.
- src/js/restart-timing.js + app.js: UI-side performance.now() marks
  (script.load, ws.open, ws.first_msg, ui.first_state, window.load),
  posted once to the controller.
- scripts/bbctrl.service: stdout/stderr -> journal so TRACE lines are
  visible via journalctl -u bbctrl. (Was StandardOutput=null.)

Revert: git revert this commit. To disable at runtime without
reverting, set BBCTRL_TRACE=0 in the bbctrl service environment.

src/py/bbctrl/Web.py

@@ -885,6 +885,31 @@ class RemoteDiagnosticsHandler(bbctrl.APIHandler):
                     'message': e.reason or "Unknown"
                 })
 
+class TimingHandler(bbctrl.APIHandler):
+    """Return the bbctrl process startup timeline as JSON.
+
+    Includes monotonic-anchored events from bbctrl.Trace, the wall
+    clock anchor (so the timeline can be aligned with journalctl /
+    systemd-analyze output), and the most recent UI-side timing
+    payload posted by the browser.
+    """
+    def get(self):
+        import bbctrl.Trace as _T
+        self.write_json(_T.timeline())
+
+
+class UITimingHandler(bbctrl.APIHandler):
+    """Browser posts its performance.now() marks here once per load."""
+    def put_ok(self):
+        import bbctrl.Trace as _T
+        # self.json is parsed in APIHandler.prepare()
+        try:
+            _T.set_ui_timing(self.json)
+            _T.mark('ui.posted_timing',
+                    marks=len(self.json.get('marks', []) or []))
+        except Exception: pass
+
+
 # Base class for Web Socket connections
 class ClientConnection(object):
     def __init__(self, app):
@@ -960,6 +985,12 @@ class SockJSConnection(ClientConnection, sockjs.tornado.SockJSConnection):
             ip = info.ip
             if 'X-Real-IP' in info.headers: ip = info.headers['X-Real-IP']
             self.app.get_ctrl(id).log.get('Web').info('Connection from %s' % ip)
+            try:
+                if not getattr(self.app, '_first_ws', False):
+                    self.app._first_ws = True
+                    import bbctrl.Trace as _T
+                    _T.mark('ws.first_open', ip=ip)
+            except Exception: pass
             super().on_open(id)
 
 
@@ -989,6 +1020,8 @@ class Web(tornado.web.Application):
 
         handlers = [
             (r'/websocket', WSConnection),
+            (r'/api/diag/timing', TimingHandler),
+            (r'/api/diag/timing/ui', UITimingHandler),
             (r'/api/log', LogHandler),
             (r'/api/message/(\d+)/ack', MessageAckHandler),
             (r'/api/bugreport', BugReportHandler),