[optimizer] debugging: capture id and plan in a span when lowering MIR

[mgree]

We've seen some one-shot selects with joins that are missing arrangements; adding a span here should let us see more about the issue.

Motivation

• This PR fixes a previously unreported bug.

Logs are triggering a soft_assert_or_log in lowering.rs, but it's not clear what the high-level query is that's causing the issue.

Tips for reviewer

Checklist

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