Made File.Handle a non-copyable struct in the library to adopt Swift 6.4.

Sources/PlaydateKit/File/Classes/Handle.swift

@@ -1,11 +1,13 @@
 internal import CPlaydate
 
 extension File {
-    /// An open file. Wraps `SDFile`. The file is closed on deinit if it has
-    /// not been closed explicitly.
-    public final class Handle {
+    /// An open file. Wraps `SDFile`. The file is closed when the handle goes
+    /// out of scope, unless it was closed explicitly with `close()`.
+    ///
+    /// The handle is non-copyable: it has a single owner, so it cannot be
+    /// used after `close()` and no heap allocation backs it.
+    public struct Handle: ~Copyable {
         let pointer: UnsafeMutableRawPointer
-        private var isClosed = false
 
         /// Opens the file at `path`.
         public init(path: String, mode: Options) throws(PlaydateError) {
@@ -17,15 +19,16 @@ extension File {
         }
 
         deinit {
-            if !isClosed {
-                _ = fileAPI.pointee.close.unsafelyUnwrapped(pointer)
-            }
+            _ = fileAPI.pointee.close.unsafelyUnwrapped(pointer)
         }
 
-        /// Closes the file. Further operations are invalid.
-        public func close() throws(PlaydateError) {
-            guard !isClosed else { return }
-            isClosed = true
+        /// Closes the file, consuming the handle.
+        // `@export(interface)` lets `discard` compile in Embedded Swift with
+        // the 6.4 release toolchain; later toolchains accept it without.
+        @export(interface)
+        public consuming func close() throws(PlaydateError) {
+            let pointer = self.pointer
+            discard self
             if fileAPI.pointee.close.unsafelyUnwrapped(pointer) != 0 { throw lastFileError() }
         }