vgpu in-guest producer in-tree, release CI, flexible vmie discovery

- src/si/vgpu-stream: in-guest vgpu producer built as a Windows cross-compiled target (if(WIN32))
- .gitea: release workflow — cross-build the agent and build/publish the deb against system vmie
- cmake/makefile: resolve vmie from a source tree (LIBVMIE_PATH) or installed libvmie-dev

src/si/vgpu-stream/win32/capture_dda.h

@@ -0,0 +1,10 @@
+#ifndef VGPU_CAPTURE_DDA_H
+#define VGPU_CAPTURE_DDA_H
+
+/* capture_dda.h — DXGI Desktop Duplication capture backend (win32). */
+
+#include "ctx.h"           /* win32 vgpu_ctx */
+
+int dda_start(vgpu_ctx* ctx, int fps);
+
+#endif /* VGPU_CAPTURE_DDA_H */