feat(alarms): HiLo trigger type with per-band level, hysteresis, messages, overrides

Adds a new HiLo alarm trigger type with four configurable setpoints
(LoLo / Lo / Hi / HiHi). Each setpoint carries an optional priority,
deadband (for hysteresis), and operator message. The site runtime emits
AlarmStateChanged with an AlarmLevel field so consumers can differentiate
warning vs critical bands.

Plumbing:
  - new AlarmLevel enum + AlarmStateChanged.Level/Message init properties
  - AlarmTriggerEditor (Blazor) gets a HiLo render with severity tinting
  - AlarmTriggerConfigCodec extracted from the editor for testability
  - sitestream.proto carries level + message over gRPC
  - SemanticValidator enforces numeric attribute, setpoint ordering,
    non-negative deadband
  - on-trigger scripts get an Alarm global (Name/Level/Priority/Message)
    so notification routing can branch by severity
  - per-instance InstanceAlarmOverride entity + EF migration + flattening
    step + CLI commands; HiLo overrides merge setpoint-by-setpoint, binary
    types whole-replace
  - DebugView shows a Level badge + per-band message tooltip
  - App.razor auto-reloads on permanent Blazor circuit failure
  - docker/regen-proto.sh automates the proto regen workflow (the linux/arm64
    protoc segfault means generated files are checked in for now)

src/ScadaLink.Commons/Messages/Streaming/AlarmStateChanged.cs

@@ -7,4 +7,23 @@ public record AlarmStateChanged(
     string AlarmName,
     AlarmState State,
     int Priority,
-    DateTimeOffset Timestamp);
+    DateTimeOffset Timestamp)
+{
+    /// <summary>
+    /// Severity level when <see cref="State"/> is <see cref="AlarmState.Active"/>.
+    /// Always <see cref="AlarmLevel.None"/> for binary trigger types
+    /// (ValueMatch, RangeViolation, RateOfChange); set by the HiLo trigger
+    /// type to one of Low/LowLow/High/HighHigh based on the crossed setpoint.
+    /// Added as an init-property so existing positional constructors still
+    /// work — message contract evolves additively.
+    /// </summary>
+    public AlarmLevel Level { get; init; } = AlarmLevel.None;
+
+    /// <summary>
+    /// Optional per-band operator message (e.g., "Coolant critically low —
+    /// shut down"). Set by HiLo triggers when the per-setpoint message is
+    /// configured; otherwise empty. Notification routing and UI tooltips may
+    /// surface this to operators.
+    /// </summary>
+    public string Message { get; init; } = string.Empty;
+}