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/region.h

@@ -0,0 +1,28 @@
+#ifndef VGPU_REGION_H
+#define VGPU_REGION_H
+
+/* region.h — win32 pinned contract region (resolves blocks for the region-view). */
+
+#include <stdint.h>
+#include "vgpu_stream.h"   /* public contract: blocks, offsets, slot geometry */
+
+/*
+ * One contiguous 2 MiB-aligned pinned region holding the full contract:
+ * producer block (page 0), control block (page 1), then SLOT_COUNT frame slots
+ * starting at VGPU_RING_OFFSET. Object = memory: the region owns the mapping,
+ * its lifetime is the mapping's lifetime. No hidden global state.
+ */
+typedef struct {
+    void*            os_base;   /* raw allocation base (for free) */
+    uint8_t*         base;      /* 2 MiB-aligned region base (== contract origin) */
+    uint64_t         os_total;  /* bytes reserved at os_base */
+    vgpu_producer_t* producer;  /* base + VGPU_PRODUCER_OFFSET */
+    vgpu_control_t*  control;   /* base + VGPU_CONTROL_OFFSET  */
+    uint8_t*         ring;      /* base + VGPU_RING_OFFSET      */
+} vgpu_region_t;
+
+/* Returns 0 on success, non-zero on failure (region zeroed on failure). */
+int  vgpu_region_create(vgpu_region_t* out);
+void vgpu_region_destroy(vgpu_region_t* r);
+
+#endif /* VGPU_REGION_H */