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.

scripts/bbctrl.service

@@ -7,7 +7,11 @@ User=root
 ExecStart=/usr/local/bin/bbctrl -l /var/log/bbctrl.log
 WorkingDirectory=/var/lib/bbctrl
 Restart=always
-StandardOutput=null
+# StandardOutput was 'null'. Set to 'journal' so TRACE lines emitted by
+# bbctrl.Trace are visible via `journalctl -u bbctrl`. Bbctrl still
+# writes its own log via -l above; this only affects stdout/stderr.
+StandardOutput=journal
+StandardError=journal
 Nice=-10
 KillMode=process