Fix Buffer size setting never being applied; add Safety Switch control type

- Buffer size (samples) in Settings > Acquisition was captured into the
  settings dict but never read anywhere — SignalBuffer always used the
  hardcoded MAX_BUFFER=20_000 constant (which happens to match the
  dialog's default, hence it looking like the value "reverts"). Add
  AcquisitionEngine.set_buffer_size(), wired in on startup and on Settings
  apply; existing buffers are rebuilt at the new size in place, keeping
  their most recent samples.

- New "Safety Switch" control type: an On/Off Switch that's disabled
  outright until acquisition is running, with a lock icon shown while
  disabled. AcquisitionEngine gains a run_state_changed(bool) signal
  (emitted from start()/stop()); ControlWidget gains a
  _check_safety_lock() hook (no-op by default, called uniformly by
  ControlPanel on every widget) that SafetySwitch overrides. engine is
  now threaded through to every ControlWidget for this. Persists via
  ControlSpec.control_type alone — no schema change needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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