exclude-newer-index: per-index cooldown periods #19749

Summary

The scientific Python ecosystem heavily relies on nightly builds of a small set of crucial packages (numpy, scipy, matplotlib...) for testing. These "nightly channels" are implemented in the form of a simple index.
uv is already able to resolve across multiple registries, but I find it compounds non-ideally with cooldown periods; a typical recommendation for a cooldown period is a week, with is longer than the life cycle of many artifacts from, e.g., https://pypi.anaconda.org/scientific-python-nightly-wheels/simple/
Currently, users can either

• disable cooldowns entirely for a selection of jobs that install from multiple registries. This is obviously not a very secure choice, especially in the absence of a lock file.
• whitelist the small set of crucial dependencies that we want nightlies from via exclude-newer-package, but this will also override cooldown periods for CI jobs that run against PyPI only.

I think a better solution would be to fine tune cooldown periods per index rather than packages. In short, I'd like to have a exclude-newer-index parameter. The obvious next question is how would this option compound with exclude-newer(-package). I think it'd make sense that it overrides a global exclude-newer just as exclude-newer-package already does, but it should also itself be overridden by exclude-newer-package. Schematically:
exclude-newer-package > exclude-newer-index > exclude-newer

Example

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