perf(template-engine): cache Expression-trigger compile verdicts under the trigger-surface key — unchanged expressions compile once per process (plan R2-05 T3)

src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Validation/ValidationService.cs

@@ -530,19 +530,27 @@ public class ValidationService
     /// <returns>A human-readable error message if the expression is invalid; <c>null</c> if well-formed.</returns>
     internal static string? CheckExpressionSyntax(string expression)
     {
-        // Authoritative forbidden-API verdict first. Report ALL violations, not just
-        // the first, so an operator fixing one forbidden API isn't surprised by a
-        // second on the next deploy attempt (mirrors ScriptCompiler.TryCompile).
-        var violations = ScriptTrustValidator.FindViolations(expression);
-        if (violations.Count > 0)
-            return $"uses forbidden API: {string.Join("; ", violations)}";
+        // Memoise the verdict under the TRIGGER surface key (see ScriptCompileVerdictCache):
+        // the verdict is a pure function of expression + policy + TriggerCompileSurface, and
+        // it is the exact hot-path cost N1 flagged — every staleness sweep / import probe of
+        // an Expression-triggered config re-ran a full trust compilation + script compile.
+        var (_, error) = ScriptCompileVerdictCache.GetOrAdd(nameof(TriggerCompileSurface), expression, () =>
+        {
+            // Authoritative forbidden-API verdict first. Report ALL violations, not
+            // just the first, so an operator fixing one forbidden API isn't surprised
+            // by a second on the next deploy attempt (mirrors ScriptCompiler.TryCompile).
+            var violations = ScriptTrustValidator.FindViolations(expression);
+            if (violations.Count > 0)
+                return (false, $"uses forbidden API: {string.Join("; ", violations)}");
 
-        // Real compile of the bare boolean expression against the trigger globals.
-        var errors = RoslynScriptCompiler.Compile(expression, typeof(TriggerCompileSurface));
-        if (errors.Count > 0)
-            return $"is not a valid expression: {string.Join("; ", errors)}";
+            // Real compile of the bare boolean expression against the trigger globals.
+            var errors = RoslynScriptCompiler.Compile(expression, typeof(TriggerCompileSurface));
+            if (errors.Count > 0)
+                return (false, $"is not a valid expression: {string.Join("; ", errors)}");
 
-        return null;
+            return (true, (string?)null);
+        });
+        return error;
     }
 
     /// <summary>