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>

1 files changed, 55 insertions, 4 deletions

diff --git a/ui/control_panel.py b/ui/control_panel.py
index 830b985..e938ceb 100644
--- a/ui/control_panel.py
+++ b/ui/control_panel.py
@@ -126,7 +126,8 @@ class ControlWidget(QFrame):
                  registry: Optional[DeviceRegistry] = None,
                  active_low: bool = False,
                  processor=None,
-                 on_action_script: str = ""):
+                 on_action_script: str = "",
+                 engine=None):
         super().__init__()
         self.title      = title
         self.icon       = icon
@@ -135,6 +136,9 @@ class ControlWidget(QFrame):
         self.registry   = registry
         self.active_low = active_low
         self._processor = processor
+        self.engine     = engine   # AcquisitionEngine — only SafetySwitch uses this
+                                    # today (run_state_changed-driven interlock), but
+                                    # every widget gets it for future control types.
         self._on_action_fn = None
         if on_action_script.strip():
             self._compile_action(on_action_script)
@@ -142,6 +146,14 @@ class ControlWidget(QFrame):
         self._build_frame()
         self._stale = None
         self._check_stale()
+        self._check_safety_lock()
+
+    def _check_safety_lock(self):
+        """No-op for regular controls — overridden by SafetySwitch to
+        enable/disable itself based on self.engine.is_running(). Called
+        uniformly on every widget from ControlPanel._on_run_state_changed()
+        so callers never need an isinstance check."""
+        pass
 
     def _compile_action(self, script: str):
         try:
@@ -349,6 +361,37 @@ class OnOffSwitch(ControlWidget):
 
 
 # ══════════════════════════════════════════════════════════════════════════════
+#  Safety Switch — On/Off Switch, but disabled until acquisition is running
+# ══════════════════════════════════════════════════════════════════════════════
+
+class SafetySwitch(OnOffSwitch):
+    """On/Off Switch that's disabled (can't be pressed at all) until
+    acquisition is running — physical safety interlock so a critical/
+    dangerous output can't be armed before the system starts actively
+    monitoring, or left armable after it's stopped. Shows a lock icon in
+    its header while disabled. Wired to AcquisitionEngine.run_state_changed
+    via ControlPanel._on_run_state_changed(), which calls every widget's
+    _check_safety_lock() uniformly — see ControlWidget's base (no-op)
+    implementation."""
+
+    def __init__(self, title: str = "Safety", icon: str = "🔒", **kw):
+        super().__init__(title, icon=icon, **kw)
+        self._lock_lbl = QLabel("🔒")
+        self._lock_lbl.setObjectName("controlWidgetSafetyLock")
+        self._lock_lbl.setToolTip("Locked — only active while acquisition is running")
+        self._header_layout.addWidget(self._lock_lbl)
+        self._check_safety_lock()
+
+    def _check_safety_lock(self):
+        if not hasattr(self, "_lock_lbl"):
+            return   # base ControlWidget.__init__ calls this before our own
+                      # __init__ (and OnOffSwitch's _build_body()) has run
+        running = bool(self.engine and self.engine.is_running())
+        self._btn.setEnabled(running)
+        self._lock_lbl.setVisible(not running)
+
+
+# ══════════════════════════════════════════════════════════════════════════════
 #  Motor Control
 # ══════════════════════════════════════════════════════════════════════════════
 
@@ -738,11 +781,16 @@ class ControlPanel(QWidget):
         self._build()
         if engine is not None:
             engine.device_status_changed.connect(self._on_device_status_changed)
+            engine.run_state_changed.connect(self._on_run_state_changed)
 
     def _on_device_status_changed(self, device_id: str, status: str):
         for w in self._widgets:
             w._check_stale()
 
+    def _on_run_state_changed(self, running: bool):
+        for w in self._widgets:
+            w._check_safety_lock()
+
     # ── Layout ───────────────────────────────────────────────────────────────
 
     def _build(self):
@@ -793,7 +841,7 @@ class ControlPanel(QWidget):
 
     def _add_widget_from_spec(self, spec):
         """Instantiate a ControlWidget from a ControlSpec and add to panel."""
-        widget = _build_widget_from_spec(spec, self.registry, self.processor)
+        widget = _build_widget_from_spec(spec, self.registry, self.processor, self.engine)
         if widget is None:
             return
         self._wire_widget(widget, spec)
@@ -897,7 +945,7 @@ class ControlPanel(QWidget):
         self._widgets.pop(idx); self._specs.pop(idx)
 
         # Insert new one at same position
-        new_widget = _build_widget_from_spec(new_spec, self.registry, self.processor)
+        new_widget = _build_widget_from_spec(new_spec, self.registry, self.processor, self.engine)
         if new_widget is None:
             return
         self._wire_widget(new_widget, new_spec)
@@ -931,7 +979,7 @@ class ControlPanel(QWidget):
 # ── Factory — build a ControlWidget from a ControlSpec ─────────────────────────
 
 def _build_widget_from_spec(spec, registry: DeviceRegistry,
-                            processor=None) -> Optional[ControlWidget]:
+                            processor=None, engine=None) -> Optional[ControlWidget]:
     """Instantiate the right ControlWidget subclass from a ControlSpec."""
     # Base kwargs — icon is NOT included here; it's passed explicitly below
     # so subclasses never get a double-value collision.
@@ -942,12 +990,15 @@ def _build_widget_from_spec(spec, registry: DeviceRegistry,
         active_low=getattr(spec, "active_low", False),
         processor=processor,
         on_action_script=getattr(spec, "on_action_script", ""),
+        engine=engine,
     )
     t   = spec.control_type
     ttl = spec.title
     ico = spec.icon
     if t == "On/Off Switch":
         return OnOffSwitch(title=ttl, icon=ico, **base_kw)
+    elif t == "Safety Switch":
+        return SafetySwitch(title=ttl, icon=ico, **base_kw)
     elif t == "Motor Control":
         return MotorControl(title=ttl, icon=ico, max_rpm=spec.max_rpm, **base_kw)
     elif t == "PWM Output":