ci: execute the public crate suite on native Linux ARM64

[la3lma]

Closes #43

What changed

• add a native ubuntu-24.04-arm / aarch64-unknown-linux-gnu public-crate lane while retaining the existing independent cross-compile check
• run the same all-feature public crate tests and warnings-denied rustdoc as the other native platforms
• run the bounded release soak on native Linux ARM64
• record AVX2 availability explicitly in host-identity JSON and job summaries; the existing SIMD test proves unavailable kernels return None without altering output
• document preview-runner retry/fallback policy and keep the compatibility promotion pending until hosted evidence is retained

Local validation

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

Hosted timings are diagnostics only and will not be used as performance evidence.

[la3lma]

Hosted evidence is complete for the final head 176f56a678dc802eb48c3a5f1b25fe5e52850901.

• Release readiness run 31332482762: native Linux ARM64 public-crate lane passed in 29 seconds; all other release-readiness jobs passed.
• CI run 31332482763: quality gate and both performance tripwires passed.
• The retained native ARM artifact identifies Ubuntu 24.04.4 LTS ARM64, Rust host aarch64-unknown-linux-gnu, and avx2_available: false.
• The all-feature public crates, rustdoc, and bounded release soak all executed natively.

The compatibility and pre-release records are now bound to the earlier successful retained run, while this final head independently re-proves the workflow and repository gates.