What problem does this solve or what need does it fill?

Query::single and Query::single_mut are convenient lazy helpers with a nasty side effect: they panic unless exactly one matching entity is found!

While this is fine, in theory, for polished, tiny example code, it teaches bad habits to our users.

What solution would you like?

We should swap over all panicking uses over to the shiny new Single system param.

In rare cases, swapping to Query::get_single or even Option<Single<&Component>> might be preferred, to allow for more detailed error handling or to ensure the system runs to do secondary work even if the single entity it's looking for isn't found.

What alternative(s) have you considered?

We could remove these methods entirely, per #14275. This is a much larger and more controversial change. Even if this is chosen, this migration needs to be done anyways, so we might as well do it now.

Additional context

The nascent bevy_cli project introduced a lint to check for this class of error: TheBevyFlock/bevy_cli#95. At some point, we'll want to run that Bevy-specific linter in CI for this project, but it's not ready yet.