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.TemplateEngine/Validation/ValidationService.cs

@@ -232,4 +232,50 @@ public class ValidationService
         }
         return null;
     }
+
+    /// <summary>
+    /// Extracts the four HiLo setpoints from a trigger configuration JSON.
+    /// Any unset (or non-numeric) setpoint comes back as <c>null</c>. Returns
+    /// all-nulls on malformed JSON — callers should treat that as "nothing to
+    /// validate" and let other checks surface the deeper problem.
+    /// </summary>
+    internal static HiLoSetpoints ExtractHiLoSetpoints(string triggerConfigJson)
+    {
+        try
+        {
+            using var doc = JsonDocument.Parse(triggerConfigJson);
+            var root = doc.RootElement;
+            return new HiLoSetpoints(
+                LoLo: ReadDouble(root, "loLo"),
+                Lo:   ReadDouble(root, "lo"),
+                Hi:   ReadDouble(root, "hi"),
+                HiHi: ReadDouble(root, "hiHi"),
+                LoLoDeadband: ReadDouble(root, "loLoDeadband"),
+                LoDeadband:   ReadDouble(root, "loDeadband"),
+                HiDeadband:   ReadDouble(root, "hiDeadband"),
+                HiHiDeadband: ReadDouble(root, "hiHiDeadband"));
+        }
+        catch (JsonException)
+        {
+            return new HiLoSetpoints(null, null, null, null, null, null, null, null);
+        }
+    }
+
+    private static double? ReadDouble(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var p)) return null;
+        return p.ValueKind switch
+        {
+            JsonValueKind.Number => p.GetDouble(),
+            JsonValueKind.String when double.TryParse(p.GetString(),
+                System.Globalization.NumberStyles.Float,
+                System.Globalization.CultureInfo.InvariantCulture, out var v) => v,
+            _ => null
+        };
+    }
 }
+
+internal readonly record struct HiLoSetpoints(
+    double? LoLo, double? Lo, double? Hi, double? HiHi,
+    double? LoLoDeadband = null, double? LoDeadband = null,
+    double? HiDeadband = null, double? HiHiDeadband = null);