Drop the macOS release target and macOS CI jobs #1666

Parent

#1665

What to build

Remove native macOS (aarch64-apple-darwin) from the release pipeline and remove all macos-26 jobs and profiles from CI so that thirdshift builds, tests, and releases on Linux runners only:

• Remove aarch64-apple-darwin and its macos-26 custom runner mapping from the dist configuration, leaving only the static Linux targets (x86_64-unknown-linux-musl and aarch64-unknown-linux-musl added by Release an ARM Linux build so the installer and thirdshift update work on aarch64 Linux #1663), and regenerate the release workflow through the repository's release generator script so no macOS build job or installer entry is generated.
• Remove the macOS CI plan, gate, shards, aggregate check (macos-plan, test-macos-gate, test-macos-shard, test-macos), and test:macos label handling from the CI workflow, while preserving the Linux x86_64 and aarch64 CI jobs and concurrency rules.
• Remove the macos-gate profile from the nextest configuration.
• Update the CI performance and local testing documentation so the active job bounds, runner policy, and local test notes reflect the Linux-only CI workflow (while leaving historical research notes under docs/research/ untouched).

Acceptance criteria

• The dist configuration lists only x86_64-unknown-linux-musl and aarch64-unknown-linux-musl, the regenerated release workflow has no aarch64-apple-darwin or macos-26 job, and the release generator freshness check and unit tests pass.
• The CI workflow has no macos-plan, test-macos-gate, test-macos-shard, or test-macos jobs, and the nextest configuration has no macos-gate profile.
• The CI performance and local testing documentation reflect the Linux-only CI workflow without active macos-gate or macos-26 job tables.
• scripts/affected-tests main passes.