fix: embed Windows executable icon

BUILD.md

@@ -21,7 +21,9 @@ The project uses pure Go and sets `CGO_ENABLED=0`. No C compiler, Windows SDK,
 ```text
 cmd/fancywin/                          CLI entry point
 assets/fancywin.png                    Full-resolution generated icon source
+assets/fancywin.ico                    Windows executable icon source
 internal/assets/                       Embedded runtime image assets
+cmd/fancywin/rsrc_windows_*.syso       Embedded Windows executable icon resources
 internal/config/                       YAML model, loading, and validation
 internal/layout/                       Platform-neutral zone geometry
 internal/platform/platform_windows.go  Win32 backend
@@ -65,6 +67,12 @@ work without a second binary. Normal execution calls `FreeConsole` and operates
 through the notification-area icon. `--debug` retains the console, while
 startup failures in detached mode are shown with `MessageBoxW`.
 
+The architecture-specific `.syso` files beside the `main` package embed the
+same FancyWin artwork used by the notification-area icon as the executable's
+Windows icon resource. The Go toolchain selects and links the matching resource
+automatically, so Explorer and shortcuts display the FancyWin icon without any
+additional build tools.
+
 ## Cross-compiling
 
 The standard x64 build can be produced from the repository root with:
@@ -158,10 +166,12 @@ Test both x64 and ARM64 where hardware is available:
     is used for subsequent moves.
 12. Confirm the custom icon is crisp in the normal and overflow notification
     areas at 100%, 150%, and 200% scaling.
-13. Select each named layout from the tray, confirm the check mark and overlay
+13. Create a shortcut to `fancywin.exe` and confirm Explorer displays the same
+    FancyWin artwork instead of the default application icon.
+14. Select each named layout from the tray, confirm the check mark and overlay
     change immediately, and verify `active_layout` changes without disturbing
     YAML comments or layout formatting.
-14. Right-click the tray icon, select `Exit`, and confirm the process and icon
+15. Right-click the tray icon, select `Exit`, and confirm the process and icon
     both disappear cleanly.
 
 ## Architecture