repo: consolidate release docs and ci

[arduano]

Summary

• align release-facing docs and crate metadata with the merged math/test state
• modernize the GitHub Actions workflow away from deprecated actions-rs usage
• keep the release branch honest about current SIMD math expectations and platform support

Local validation

• cargo fmt --check
• cargo clippy --all --all-targets -- --deny warnings
• cargo clippy --all --all-targets --target aarch64-unknown-linux-gnu -- --deny warnings
• cargo test -q
• cargo test --release
• cargo test --release --target wasm32-wasip1 --no-run
• CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS='-C target-feature=+avx512f,+avx512bw,+avx512dq' cargo test --tests --target x86_64-unknown-linux-gnu --no-run
• CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS='-C target-feature=+avx512f,+avx512bw,+avx512dq' cargo test --release --lib --target x86_64-unknown-linux-gnu --no-run

Notes

• local wasm execution is not available on this host because wasmtime is not installed; CI should perform the real wasm32-wasip1 test run via the workflow-installed runner