[2026-03 CWG Motion 9] P3726R2 Adjustments to Union Lifetime Rules

[eisenwave]

Fixes NB DE-087 (C++26 CD).
Fixes #8830

Also fixes cplusplus/papers#2356
Also fixes https://github.com/cplusplus/nbballot/issues/664
Also fixes cplusplus/papers#2418

[brevzin]

Dunno if we also want to link the other NB comments here (which I forgot to mention in the paper):

• US 48-086, which is marked a direct dupe of the German one.
• FR-006-088, which actually asked to revert all of p3074
• PL-003, which asked to just revert part of p3074 (which this paper does, but goes further).

[eisenwave]

Dunno if we also want to link the other NB comments here (which I forgot to mention in the paper):

Thanks for the list. I'll leave that up to Thomas. I'm not sure if we want dupes and already closed issues to be referenced in the commit message and/or in the issue.

[jensmaurer]

I think we want links to "accepted" NB comments in the commit and in the pull request where the commit contributes to resolving the NB comment. That also includes duplicates (because national bodies ostensibly don't know about each other's comment), but not "rejected; we're going in another direction" issues.