input: crash binding can configure which thread to crash

src/termio/Thread.zig

@@ -268,6 +268,7 @@ fn drainMailbox(
 
         log.debug("mailbox message={}", .{message});
         switch (message) {
+            .crash => @panic("crash request, crashing intentionally"),
             .change_config => |config| {
                 defer config.alloc.destroy(config.ptr);
                 try io.changeConfig(data, config.ptr);

src/termio/message.zig

@@ -29,6 +29,10 @@ pub const Message = union(enum) {
         padding: renderer.Padding,
     };
 
+    /// Purposely crash the renderer. This is used for testing and debugging.
+    /// See the "crash" binding action.
+    crash: void,
+
     /// The derived configuration to update the implementation with. This
     /// is allocated via the allocator and is expected to be freed when done.
     change_config: struct {