feat(core): add Cargo benches/ to EXACT_DIR_NAMES

Rust's official Cargo convention is `benches/` (plural, trailing `s`),
matching the target type documented at
https://doc.rust-lang.org/cargo/reference/cargo-targets.html — every
`.rs` file under that directory compiles as a separate benchmark
binary. The existing list picked up the C++-style `bench` / `benchmark`
/ `benchmarks` variants but missed the Rust one, so Cargo workspaces
were silently getting their criterion / test::Bencher sources indexed.

Add the plural form as a case-insensitive exact-name match. Also useful
to any language that adopts the naming (some Go and Zig projects use
`benches/` too).