Deny internal exectuion order ambiguities in CI

[alice-i-cecile]

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

As discussed in #7383, internal execution order ambiguities are genuinely bad.

They often represent subtle bugs, break determinism for end users, and result in noisy logs for users.

What solution would you like?

1. Recreate the enum from Improvements to execution order ambiguity reporting #4299 to create configurable levels of system execution order ambiguity reporting / strictness first.
2. Run the example from [Merged by Bors] - Reduce internal system order ambiguities, and add an example explaining them #7383 in CI.
3. Ensure that this example panics if any ambiguities are found

To actually get this to pass, we need more powerful tools to resolve or ignore ambiguities between plugins that aren't aware of them.
Once #7267 is merged, we should have those.

Then, add some form of IntegrationPlugin to DefaultPlugins that uses system set configuration + feature flags to resolve or ignore cross-plugin ambiguities.

What alternative(s) have you considered?

Do this manually before every release 🥲

We could just ignore ambiguities on the offending systems completely, but that's definitely incorrect for important systems like UI layout and animation.

[Vrixyz]

#13950 adds a dedicated example for ambiguity detection, which is run on CI.

You'll notice on the PR a list of disabled schedule labels, which you can clear to get a full list of current ambiguities.