.github, docs, exercises: bump Zig from 0.14.1 to 0.15.0

[ee7]

This PR is intended to stay as draft until the 0.15.0 release, at which point I'll update this PR to use plain 0.15.0.

I'm just making this PR to get the tests passing in advance, and as the corresponding PR for exercism/zig-test-runner#127.

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This should be an exciting release for Zig, because it's expected to make the x86_64 backend the default for debug builds. That'll speed up things a lot for both those using Exercism locally and in the browser.

See:

• The relevant part of YouTube: Zig Roadmap 2026 (streamed 2025-07-03).
• The 2025-06-08 Zig devlog post announcing the change for the master branch: Self-Hosted x86 Backend is Now Default in Debug Mode.
• The blocking issues in the 0.15.0 milestone. A previously stated (although at the start of July) tentative release date for Zig 0.15.0 was 2025-08-01, so the release is probably not too far off.

Edit 2025-07-31: The 0.15.0 milestone target date is now 2025-08-12.

As a sample size of 1, the Linux workflow's testing step took:

• 36s with this PR.
• 2m18s in the previous run on main.

So thats a roughly 4x speedup, which matches the speedup advertised above.

Closes: #342

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To-do:

• Wait for 0.15.0 release
• Update this PR with the changes required by the 0.15.0 release.
• Ensure that the corresponding PR in exercism/zig-test-runner is ready to merge.