chore: Bump mlxcel-core to Rust edition 2024 to match the root crate

[inureyes]

Summary

The root mlxcel crate is on edition 2024, but src/lib/mlxcel-core/Cargo.toml is still on edition 2021. Align mlxcel-core (and any other workspace members still on 2021) to edition 2024 so the workspace is consistent and picks up the 2024 defaults/lints.

Why now

PR #270 added mlxcel_core::ensure_persistent_ptx_cache(), which calls std::env::set_var. In edition 2021 set_var is safe, so it compiles without unsafe; in edition 2024 set_var/remove_var are unsafe. When mlxcel-core moves to 2024 that call must be wrapped in unsafe { ... } with a short safety comment (it runs once at startup before any worker threads spawn). This is the one known required touch-up, but the migration should be done deliberately rather than ad hoc.

Scope and risks

Edition 2024 is more than set_var. A careful migration should account for:

• std::env::set_var/remove_var become unsafe (wrap the call in ensure_persistent_ptx_cache, plus any other call sites).
• gen becomes a reserved keyword (rename any identifier named gen).
• RPIT (return-position impl Trait) lifetime capture rule change (may need use<...> bounds).
• if let temporary scope / drop-timing change.
• unsafe extern blocks.
• Other 2024 default changes surfaced by the migration.

mlxcel-core is large and carries the cxx FFI bridge, so the surface is non-trivial and some changes only appear in cfg-gated code (CUDA, Metal).

Approach

• Run cargo fix --edition on mlxcel-core (and any other 2021 members), then bump edition = "2024" in their Cargo.toml.
• Review the auto-applied changes; hand-fix anything cargo fix cannot.
• Verify with a full build on every backend that matters, including the CUDA path (edition changes can hide in #[cfg(feature = "cuda")] code that a macOS build never compiles), plus cargo clippy --all-targets -D warnings and cargo fmt --check.

Acceptance criteria

• All workspace crates are on edition 2024.
• ensure_persistent_ptx_cache (and any other set_var/remove_var call sites) wrap the call in unsafe { ... } with a safety comment.
• cargo build succeeds on macOS (metal,accelerate) and on Linux CUDA; clippy and fmt are clean.

Context

Spun out of #270 (CUDA NVRTC-JIT UX work), which intentionally kept mlxcel-core on edition 2021 to stay focused. This issue is the follow-up; do it after #270 merges.