termio: free resources for discarded messages

Messages can own allocated data or a derived config. Some paths (writer
thread draining, mailbox shutdown with unread messages, and queue push
failures) discarded messages without releasing those resources.

This change adds Message.deinit and uses it whenever a message is
discarded.

src/Surface.zig

@@ -872,10 +872,9 @@ fn queueIo(
         switch (msg) {
             .write_small,
             .write_stable,
-            => return,
-
-            .write_alloc => |v| {
-                v.alloc.free(v.data);
+            .write_alloc,
+            => {
+                msg.deinit();
                 return;
             },