feat(core): add Rust group to TEST_PATTERN_GROUPS with tests.rs

Rust's dominant unit-test convention is inline `#[cfg(test)] mod tests
{ ... }` blocks that cannot be excluded by filename — anyone wanting to
strip those has to post-process the source. The one file-pattern the
Rust Book (chapter 11.3) does describe is extracting that block into a
sibling `tests.rs` file next to `mod.rs` or `lib.rs`; that extracted
form is what a `**/tests.rs` rule can catch.

Integration tests already live under `tests/` (existing dir rule) and
benchmarks under `benches/` (added in the previous commit), so the
new file-pattern group is deliberately minimal — just the one glob
the community has actually converged on.

Tests: assert the Rust sub-header + pattern emission, and extend the
stable-order invariant to place Rust after Python.