feat: add Discord rich presence support

runtime/include/discord_presence.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+
+// Host implementation of Dolphin's /dev/dolphin Discord contract. The guest
+// sends only strings and big-endian integer fields; the IPC wire protocol is
+// owned here so translated game code never needs host SDK headers.
+namespace DiscordPresence {
+
+struct Activity {
+    std::string details;
+    std::string state;
+    std::string largeImageKey;
+    std::string largeImageText;
+    std::string smallImageKey;
+    std::string smallImageText;
+    int64_t startTimestamp = 0;
+    int64_t endTimestamp = 0;
+    uint32_t partySize = 0;
+    uint32_t partyMax = 0;
+};
+
+void Initialize(const std::string& basicClientId, const std::string& basicTitle);
+void SetClient(const std::string& clientId);
+void SetActivity(Activity activity);
+void Reset();
+void Shutdown();
+
+} // namespace DiscordPresence