feat(embedded-mpv): add Windows frame-copy support (#1175)

Port the embedded mpv frame-copy pipeline to Windows with WGL rendering and named shared memory. Includes packaging validation, platform gates, tests, and architecture documentation.

apps/electron-backend/native/src/embedded_mpv_frame_reader.c

@@ -7,13 +7,30 @@
  * memory cage forbids external ArrayBuffers over foreign memory.
  *
  * Plain C N-API (no node-addon-api) so it stays ABI-stable and trivial.
- * macOS and Linux (POSIX shm) — other platforms export an empty object;
- * the TypeScript side gates on platform before requiring it.
+ * macOS and Linux (POSIX shm) plus Windows (named file mapping; binding.gyp
+ * compiles this file as C++ there because MSVC's C mode has no
+ * <stdatomic.h>) — other platforms export an empty object; the TypeScript
+ * side gates on platform before requiring it.
  */
 #define NAPI_VERSION 8
 #include <node_api.h>
 
-#if defined(__APPLE__) || defined(__linux__)
+#if defined(__APPLE__) || defined(__linux__) || defined(_WIN32)
+
+#if defined(_WIN32)
+
+#include <stdint.h>
+#include <string.h>
+
+#include "../helper/frame_shm.h"
+
+/* frame_shm.h typed the fields as std::atomic in C++ mode; keep the shared
+ * atomic_load_explicit call sites below spelling-compatible. */
+using std::atomic_load_explicit;
+using std::memory_order_acquire;
+using std::memory_order_relaxed;
+
+#else
 
 #include <fcntl.h>
 #include <stdatomic.h>
@@ -26,6 +43,8 @@
 
 #include "../helper/frame_shm.h"
 
+#endif
+
 static FrameShmHeader* g_header = NULL;
 static uint8_t* g_base = NULL;
 static size_t g_size = 0;
@@ -35,7 +54,11 @@ static uint64_t now_ns(void) { return frame_shm_now_ns(); }
 
 static void unmap_current(void) {
     if (g_base) {
+#if defined(_WIN32)
+        UnmapViewOfFile(g_base);
+#else
         munmap(g_base, g_size);
+#endif
     }
     g_base = NULL;
     g_header = NULL;
@@ -68,6 +91,23 @@ static napi_value Open(napi_env env, napi_callback_info info) {
         return throw_error(env, "shm name must be a string");
     }
 
+#if defined(_WIN32)
+    char mapping_name[280];
+    frame_shm_windows_name(name, mapping_name, sizeof(mapping_name));
+    HANDLE mapping = OpenFileMappingA(FILE_MAP_READ, FALSE, mapping_name);
+    if (!mapping) return throw_error(env, "shm open failed (helper gone?)");
+    void* mapped = MapViewOfFile(mapping, FILE_MAP_READ, 0, 0, 0);
+    /* The view holds its own reference to the section object. */
+    CloseHandle(mapping);
+    if (!mapped) return throw_error(env, "shm map failed");
+    MEMORY_BASIC_INFORMATION region;
+    if (VirtualQuery(mapped, &region, sizeof(region)) == 0 ||
+        region.RegionSize < sizeof(FrameShmHeader)) {
+        UnmapViewOfFile(mapped);
+        return throw_error(env, "shm segment too small");
+    }
+    const size_t segment_size = (size_t)region.RegionSize;
+#else
     const int fd = shm_open(name, O_RDONLY, 0);
     if (fd < 0) return throw_error(env, "shm_open failed (helper gone?)");
     struct stat segment;
@@ -80,17 +120,23 @@ static napi_value Open(napi_env env, napi_callback_info info) {
         mmap(NULL, (size_t)segment.st_size, PROT_READ, MAP_SHARED, fd, 0);
     close(fd);
     if (mapped == MAP_FAILED) return throw_error(env, "mmap failed");
+    const size_t segment_size = (size_t)segment.st_size;
+#endif
 
     FrameShmHeader* header = (FrameShmHeader*)mapped;
     if (header->magic != FRAME_SHM_MAGIC ||
         header->version != FRAME_SHM_VERSION) {
-        munmap(mapped, (size_t)segment.st_size);
+#if defined(_WIN32)
+        UnmapViewOfFile(mapped);
+#else
+        munmap(mapped, segment_size);
+#endif
         return throw_error(env, "frame ring not initialized yet");
     }
 
     unmap_current();
     g_base = (uint8_t*)mapped;
-    g_size = (size_t)segment.st_size;
+    g_size = segment_size;
     g_header = header;
 
     napi_value result;
@@ -204,7 +250,7 @@ static napi_value Init(napi_env env, napi_value exports) {
     return exports;
 }
 
-#else /* neither __APPLE__ nor __linux__ */
+#else /* no frame-copy port for this platform */
 
 static napi_value Init(napi_env env, napi_value exports) {
     /* Frame-copy is not ported to this platform yet; loading this module