Update subtree/library to 2026-04-01

[github-actions]

This is an automated PR to update the subtree/library branch to the changes from 2025-11-25 (rust-lang/rust@c871d09) to 2026-04-01 (rust-lang/rust@48cc71e), inclusive.
Review this PR as usual, but do not merge this PR using the GitHub web interface. Instead, once it is approved, use git push to literally push the changes to subtree/library without any rebase or merge.

[feliperodri]

Closing — subtree/library was corrupted by web-UI merges, now repaired

Root cause (my fault): starting with #615, several "Update subtree/library" PRs (#615, #633, #634, #636, #650, #652, #654) were merged with GitHub's "Squash and merge" button. Each of those produces a synthetic single-parent commit (Update subtree/library to … (#NNN)) that does not exist in the deterministic splitsh-lite extraction of upstream's library/.

The automated PR body asks for exactly the opposite:

Review this PR as usual, but do not merge this PR using the GitHub web interface. Instead, once it is approved, use git push to literally push the changes to subtree/library without any rebase or merge.

Those 7 squash commits diverged subtree/library from the real extraction, so every subsequent auto-generated PR became a non-fast-forward and showed conflicts. It also explains the duplicate "2026-03-02" PRs (#633/#634/#636/#650/#652/#654): the job's "nothing-to-do" check greps the extraction for the branch-HEAD message, and a squash message like Update subtree/library to 2026-03-02 (#654) never matches a real upstream commit — so it kept opening a fresh PR every day.

Repair: I restored subtree/library to the clean extraction by directly pushing this branch's commits (the procedure the PR asks for). Verified the previous subtree/library HEAD tree is byte-identical to extraction commit f890ece ("Rollup merge of rust-lang#153255"), so no content was lost — the 7 squash commits were just repackaged upstream changes that the extraction already contains.

Closing this PR since its commits are now in subtree/library. I've also updated the workflow to make the "do not merge via the UI" instruction far more prominent so this can't recur.