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.c

@@ -0,0 +1,19 @@
+/* capture.c — win32 registration of the capture backends into the neutral
+ * capture seam's backend table (data-driven; no per-backend branching). */
+
+#include "capture.h"       /* neutral seam: capture_backend / capture_backends */
+#include "capture_nvfbc.h"
+#include "capture_dda.h"
+#include "capture_gdi.h"
+
+/* data-driven backend table; main selects by EYES env or first available */
+static const capture_backend g_backends[] = {
+    { "nvfbc", nvfbc_start },
+    { "dda",   dda_start   },
+    { "gdi",   gdi_start   },
+};
+
+const capture_backend* capture_backends(int* count) {
+    *count = (int)(sizeof g_backends / sizeof g_backends[0]);
+    return g_backends;
+}