[Backport release-25.05] apple-sdk_26: init

[niklaskorz]

Backport of #454747

Required for #459417

Alternative to #459537, in case we don't want to backport SDK 15.5 (which would need to go through staging-25.05, see #459537 (comment))

(didn't apply cleanly to release-25.05, unlike the other PR, so I had to resolve one conflict)

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[niklaskorz]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 459540
Commit: 0eee7e384e347bd4729fa0eee101a1abdb0da5c2

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x86_64-darwin

✅ 1 package built:

• apple-sdk_26

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aarch64-darwin

✅ 1 package built:

• apple-sdk_26

[booxter]

Thank you for this backport. I think we should avoid rebuilds and land this PR without 15.x change.