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.Communication/Protos/sitestream.proto

@@ -34,6 +34,17 @@ enum AlarmStateEnum {
   ALARM_STATE_ACTIVE = 2;
 }
 
+// Severity level for an active alarm. Binary trigger types (ValueMatch,
+// RangeViolation, RateOfChange) always emit ALARM_LEVEL_NONE. The HiLo
+// trigger type emits one of the directional values.
+enum AlarmLevelEnum {
+  ALARM_LEVEL_NONE = 0;
+  ALARM_LEVEL_LOW = 1;
+  ALARM_LEVEL_LOW_LOW = 2;
+  ALARM_LEVEL_HIGH = 3;
+  ALARM_LEVEL_HIGH_HIGH = 4;
+}
+
 message AttributeValueUpdate {
   string instance_unique_name = 1;
   string attribute_path = 2;
@@ -49,4 +60,6 @@ message AlarmStateUpdate {
   AlarmStateEnum state = 3;
   int32 priority = 4;
   google.protobuf.Timestamp timestamp = 5;
+  AlarmLevelEnum level = 6;  // ALARM_LEVEL_NONE for binary trigger types; set by HiLo.
+  string message = 7;        // Optional per-band operator message; empty when unset.
 }