Merge branch 'backend-selection' into main

- Implement runtime backend selection (Wayland, X11, Cocoa)
- Add native X11 backend with clipboard, keyboard, mouse, and placement
- Integrate multi-backend integration tests running under Weston and Xwayland
- Retain zero-hole struct packing, branchless ALU/LUT primitives, and paced event loop

1 files changed, 34 insertions, 0 deletions

diff --git a/tests/common/test_selection.c b/tests/common/test_selection.c
new file mode 100644
index 0000000..d1b3eb8
--- /dev/null
+++ b/tests/common/test_selection.c
@@ -0,0 +1,34 @@
+#define _POSIX_C_SOURCE 200809L
+#include "yasl/yasl.h"
+
+#include <assert.h>
+#include <errno.h>
+#include <stdlib.h>
+
+int main(void)
+{
+    struct yasl_display_spec spec = {.backend = (enum yasl_backend)999};
+    errno = 0;
+    assert(!yasl_display_open(NULL, &spec));
+    assert(errno == EINVAL);
+    assert(yasl_display_backend(NULL) == YASL_BACKEND_AUTO);
+#if !YASL_HAS_COCOA
+    spec.backend = YASL_BACKEND_COCOA;
+    errno = 0;
+    assert(!yasl_display_open(NULL, &spec));
+    assert(errno == ENOSYS);
+#endif
+#if !YASL_HAS_WAYLAND
+    spec.backend = YASL_BACKEND_WAYLAND;
+    errno = 0;
+    assert(!yasl_display_open(NULL, &spec));
+    assert(errno == ENOSYS);
+#endif
+#if !YASL_HAS_X11
+    spec.backend = YASL_BACKEND_X11;
+    errno = 0;
+    assert(!yasl_display_open(NULL, &spec));
+    assert(errno == ENOSYS);
+#endif
+    return 0;
+}