fix: Add compile-time warning for disabled CloudKit on Swift 6.3+

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Stijn Willems

Sources/SQLiteData/CloudKit/CloudKitAvailability.swift

@@ -0,0 +1,10 @@
+// CloudKitAvailability.swift
+// Compile-time warning for Swift 6.3+ users
+
+#if compiler(>=6.3)
+  #warning("""
+    CloudKit functionality is disabled on Swift 6.3+ due to compiler crashes.
+    See https://github.com/doozMen/sqlite-data/issues/2 for details.
+    Use Swift 6.2.x if you need CloudKit synchronization features.
+    """)
+#endif