build: add macOS ARM64 CI

[baszalmstra]

Why

PR #597 adds an osx-arm64 Pixi environment, but the macOS CI and release jobs still only run on Intel. This leaves the Apple Silicon environment untested and does not produce a native ARM64 runtime artifact.

This adds the macos-15 Apple Silicon runner alongside macos-15-intel for the Rust, doctest, and C++ jobs. It also adds a macos-aarch64 release artifact and makes the CMake package URL select that archive on Apple Silicon.

This PR is stacked on #599, which is stacked on #597.

Testing

• pixi lock --check
• actionlint .github/workflows/*.yml
• CMake configure with the local runtime path
• Rust coverage, doctests, and C++ runtime tests on macos-15

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.52%. Comparing base (381ce9a) to head (e8aab22).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #598      +/-   ##
==========================================
+ Coverage   86.05%   87.52%   +1.47%     
==========================================
  Files         268      270       +2     
  Lines       30003    32030    +2027     
==========================================
+ Hits        25818    28034    +2216     
+ Misses       4185     3996     -189     

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