ci: test public crates on macOS ARM64 and Intel

[la3lma]

Closes #44

What changed

• retain the existing native macos-15 / aarch64-apple-darwin release lane
• add a separate native macos-15-intel / x86_64-apple-darwin lane
• run identical all-feature public crate tests and warnings-denied rustdoc on both
• retain per-lane architecture, Rust host, and AVX2 availability evidence
• document that cross-architecture hosted timings are diagnostics, not comparable performance evidence
• add a narrow actionlint exception because pinned actionlint 1.7.7 predates GitHub's official macos-15-intel label

Local validation

• cargo xtask ci
• cargo test -p xtask host_identity
• cargo clippy -p xtask --all-targets -- -D warnings
• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.7 .github/workflows/release-readiness.yml
• cargo xtask verify-optimization-ledger

Compatibility promotion remains pending until both hosted identities are retained.

[la3lma]

Hosted evidence is complete.

• Release readiness run 31333000148 retained the architecture artifacts used by the compatibility record. On macOS 15.7.7, ARM64 passed in 27 seconds and recorded aarch64-apple-darwin / avx2_available: false; Intel passed in 56 seconds and recorded x86_64-apple-darwin / avx2_available: true.
• Both lanes executed identical all-feature public-crate tests and warning-denied rustdoc.
• The final head 6bab437c7881f2ba8eed043e72400260d4ab2092 re-ran cleanly in release-readiness run 31333193700 and CI run 31333193712.
• Hosted timings remain diagnostic only; no cross-architecture performance claim is made.