feat(central-ui): ParameterValueForm.ValidChanged — additive validity callback

Claude-Session: https://claude.ai/code/session_01NcMG6xXdsJbmD2wYAgkK29

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/ParameterValueForm.razor

@@ -48,6 +48,13 @@ else
     [Parameter] public Dictionary<string, object?> Values { get; set; } = new();
     [Parameter] public EventCallback<Dictionary<string, object?>> ValuesChanged { get; set; }
 
+    /// <summary>
+    /// Raised after every edit with <c>true</c> when the value tree has no parse or validation errors
+    /// (Flow Debugger script entry, design 2026-09-08 §5: the page gates Run on it). Additive; existing
+    /// callers ignore it.
+    /// </summary>
+    [Parameter] public EventCallback<bool> ValidChanged { get; set; }
+
     private IReadOnlyList<ParameterShape> _shapes = Array.Empty<ParameterShape>();
     private IReadOnlyList<InboundApiSchemaField> _topLevelFields = Array.Empty<InboundApiSchemaField>();
     private InboundApiSchema? _rootSchema;
@@ -620,8 +627,11 @@ else
     {
         Validate();
         await ValuesChanged.InvokeAsync(Values);
+        await ValidChanged.InvokeAsync(IsValid);
     }
 
+    private bool IsValid => _parseErrors.Count == 0;
+
     // Validates the whole value tree against the parsed schema and projects each
     // path-qualified error onto the input that owns it. Raw-text parse failures
     // (numeric / JSON) are added directly by the setters; schema validation adds