Added sorted log ratio path for loo

[VisruthSK]

This PR was made with assistance from Codex and Claude.

Summary

Add ps_tail_sorted_log_ratios(), a specialized helper for loo to use when the log-ratio tail has already been sorted. See stan-dev/loo#290, especially this comment for more motivation.

Removed warning in ps_tail() for too few tail draws as the aforementioned comment suggests. Also deduplicate some code in ps_tail() with a new internal helper.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if d9b320b is merged into master:

• ✔️as_draws_array: 142ms -> 143ms [-0.39%, +1.18%]
• ✔️as_draws_df: 68ms -> 68.1ms [-1.1%, +1.47%]
• ✔️as_draws_list: 158ms -> 159ms [-0.79%, +1.37%]
• ✔️as_draws_matrix: 24.7ms -> 25ms [-0.7%, +2.64%]
• ✔️as_draws_rvars: 113ms -> 113ms [-0.81%, +1.13%]
• ✔️summarise_draws_100_variables: 743ms -> 741ms [-0.86%, +0.27%]
• ✔️summarise_draws_10_variables: 81.8ms -> 81.9ms [-0.19%, +0.43%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[avehtari]

exported function might need NEWS.md item

[avehtari]

Looks good

[jgabry]

@VisruthSK is this waiting on anything else? Also does this include the performance benefit from that loo PR that we closed?

[VisruthSK]

I think I need to implement that after which it's gtg