ci: run smartcore-benches against release commit (#407)

[Mec-iS]

Summary

Adds a dedicated CI workflow that clones smartcore-benches into the runner and runs its criterion + iai-callgrind benchmarks against the released smartcore code — all in-repo, no cross-repo dispatch or PAT needed. Scoped to releases per the focused ask (#407).

Changes

• New file: .github/workflows/release-bench.yml
  • Triggers on: release: types: [published]
  • Checks out smartcore (at the release commit) + smartcore-benches (into a sibling smartcore-benches/ directory)
  • Creates a temporary .cargo/config.toml with [patch.crates-io] smartcore = { path = "../" } so the benches' smartcore = "0.6" dependency resolves to the local checkout instead of crates.io
  • Installs Valgrind (for iai-callgrind)
  • Builds benches (cargo bench --no-run)
  • Runs criterion benchmarks (--output-format bencher → bench-output/criterion.json)
  • Runs iai-callgrind benches (per-bench: iai_matmul, iai_ab, iai_svd, iai_cover_tree, iai_iterator_mut)
  • Uploads all results as artifacts (release-benchmark-results, 90-day retention)

No secrets, no PAT, no cross-repo dispatch

The benches run directly in the smartcore CI runner. The [patch.crates-io] override repoints the benches' smartcore dependency to the already-checked-out release commit — no crates.io round-trip, no repository_dispatch, no BENCHES_DISPATCH_PAT secret.

Verification

• cargo fmt --all -- --check = 0
• cargo clippy --all-features -- -Drust-2018-idioms -Drust-2024-compatibility -Dwarnings = 0
• YAML valid; pre-commit hook passed (fmt + clippy)

What this does NOT do (intentionally, per scope)

• Does not trigger benches on development pushes — that's the per-commit dispatch path (separate concern).
• Does not add iai-callgrind as a required branch-protection gate — that comes after a baseline is established.

Refs #407.