feat: add audited one-shot prepare gate

src/MBN_STOCK_WEBVIEW.LegacyBridge/LegacyPlayoutUiIntents.cs

@@ -6,6 +6,12 @@ namespace MBN_STOCK_WEBVIEW.LegacyBridge;
 public sealed record LegacyExecutePlayoutIntent(LegacyOperatorPlayoutCommand Command)
     : LegacyUiIntent;
 
+/// <summary>
+/// Process-scoped Gate A PREPARE request. The opaque capability is validated and
+/// consumed by the native launch authorization; it is never emitted in state.
+/// </summary>
+public sealed record LegacyGateAPrepareIntent(string Capability) : LegacyUiIntent;
+
 public sealed record LegacySetFadeDurationIntent(int Duration)
     : LegacyUiIntent;
 
@@ -17,6 +23,7 @@ internal static class LegacyPlayoutUiIntentParser
 {
     public static LegacyUiIntent? Parse(string type, JsonElement payload) => type switch
     {
+        "gate-a-prepare" => ParseGateAPrepare(payload),
         "prepare-playout" => ParseEmpty(
             payload,
             static () => new LegacyExecutePlayoutIntent(
@@ -43,6 +50,29 @@ internal static class LegacyPlayoutUiIntentParser
         _ => null
     };
 
+    private static LegacyUiIntent? ParseGateAPrepare(JsonElement payload)
+    {
+        if (payload.ValueKind != JsonValueKind.Object ||
+            payload.GetRawText().Length > 128 ||
+            payload.EnumerateObject().Count() != 1 ||
+            !payload.TryGetProperty("capability", out var capabilityElement) ||
+            capabilityElement.ValueKind != JsonValueKind.String)
+        {
+            return null;
+        }
+
+        var capability = capabilityElement.GetString();
+        if (capability is null || capability.Length != 64 ||
+            capability.Any(character =>
+                character is not (>= '0' and <= '9') and
+                    not (>= 'A' and <= 'F')))
+        {
+            return null;
+        }
+
+        return new LegacyGateAPrepareIntent(capability);
+    }
+
     private static LegacyUiIntent? ParseFadeDuration(JsonElement payload)
     {
         if (payload.ValueKind != JsonValueKind.Object ||