Bump Rust edition to 2024 + adopt relative idioms

[Mec-iS]

I'm submitting a

• improvement.

Summary

Bump the crate from Rust edition 2021 to edition 2024 and adopt relative modern idioms. Edition 2024 stabilized in Rust 1.85 (Feb 2025); the repo toolchain is 1.94+, so it builds locally and on CI.

Pre-migration audit (already performed, read-only)

The two edition-2024 hazards that usually bite hardest do not apply here:

• No -> impl Trait returns (zero RPIT) → no return-position-impl-trait lifetime over-capture regression.
• All dyn Trait already carry explicit + 'a lifetimes (e.g. Box<dyn ArrayView1<T> + 'a>) → no dyn-elision surprise.

Other hazards are clean or trivial:

• unsafe: only 4 unsafe { } blocks in src/linalg/basic/matrix.rs (raw-pointer iterator_mut, already tracked by tech-debt: replace unsafe raw-pointer iterator_mut in DenseMatrix (basic/matrix.rs) with safe split_at_mut approach #368). No unsafe fn / unsafe impl, so unsafe_op_in_unsafe_fn (default-warn in 2024) doesn't fire. No #[no_mangle] / #[export_name] / #[link_section] (no unsafe-attr migration).
• pub use re-exports of private modules (error::ReadingError, grid_search::…): at most unreachable_pub warnings; cargo fix --edition handles.
• cfg(coverage) / cfg(tarpaulin): not referenced in source → the new unexpected_cfgs lint won't fire from our code.
• MSRV: no rust-version declared today → edition 2024 implies MSRV ≥ 1.85. Impact is on downstream consumers of the published crate (fine for 0.x).

Plan (sub-issues below)

Sub-issue	Scope
A. Edition flip	cargo fix --edition --allow-dirty --allow-staged --all-features; set edition = "2024" in Cargo.toml; update AGENTS.md ("Rust 2021" → "Rust 2024"), README changelog line, CHANGELOG.md (breaking re: implied MSRV). Inspect the auto-fix diff before flipping the edition flag.
B. #[allow] → #[expect]	Migrate ~10 lint-suppression sites to #[expect(...)]: crate-root #![allow(...)] in lib.rs; #![allow(non_snake_case)] in cholesky.rs/svd.rs/lu.rs/auc.rs/mahalanobis.rs; #[allow(dead_code)] in distance/cosine.rs + lu.rs; #![allow(clippy::ptr_arg, clippy::needless_range_loop)] in cluster/mod.rs + cluster_helpers.rs; #![allow(clippy::wrong_self_convention)] in linalg/traits/mod.rs. Verify each lint actually fires — #[expect] errors if a suppression goes stale; keep #[allow] wherever the lint currently doesn't fire.
C. Declare MSRV	Add rust-version = "1.85" to Cargo.toml. Honest about the downstream breaking implication of edition 2024.
D. unsafe cleanup	Tracked by #368 (linked as a sub-issue below, not duplicated). Replace the 4 unsafe { } blocks in src/linalg/basic/matrix.rs with safe split_at_mut. Ships as its own PR.

Execution

• Branch edition-2024 off development (orthogonal to the improve-coverage / ci(coverage): include doctests + strict --fail-under 44 gate (Stage 0, #399) #400 effort).
• A + B + C land as one combined PR closing A/B/C and referencing this parent.
• D ships as its own PR on a separate branch closing tech-debt: replace unsafe raw-pointer iterator_mut in DenseMatrix (basic/matrix.rs) with safe split_at_mut approach #368.
• Verify per AGENTS.md: cargo fmt --all -- --check; cargo clippy --all-features -- -Drust-2018-idioms -Dwarnings; cargo test --all-features.

Do you want to work on this issue?

yes — staged PRs target development.