[gouyt13]

Summary

Move the native Apple Silicon C++ tests and ARM64 wheel builds from macos-14 to macos-15 before the announced October brownouts and November 2, 2026 retirement. Pinning macOS 15 preserves native ARM64 coverage and avoids automatic major OS changes through macos-latest.

Update the testing guide to match. The wheel deployment target remains macOS 14; job names, ABI coverage, build steps, and test steps are unchanged. GitHub lists macos-15 as ARM64 in its runner image reference.

Testing

• All workflow YAML files parse; a structural comparison confirms only the two intended runner values changed.
• No retired macOS 14 runner labels remain in workflows.
• Release automation and CI routing unit tests: 22 passed.
• git diff --check passed; reviewed the testing guide and unchanged local links.
• Native macOS builds and wheel tests require GitHub Actions and could not run on the local Linux host. PR CI will exercise the affected workflows. actionlint is unavailable locally.